nfsd: fix oops when legacy_recdir_name_error is passed a -ENOENT error

nfsd: fix oops when legacy_recdir_name_error is passed a -ENOENT error

Toralf reported the following oops to the linux-nfs mailing list:

    -----------------[snip]------------------
    NFSD: unable to generate recoverydir name (-2).
    NFSD: disabling legacy clientid tracking. Reboot recovery will not function correctly!
    BUG: unable to handle kernel NULL pointer dereference at 000003c8
    IP: [<f90a3d91>] nfsd4_client_tracking_exit+0x11/0x50 [nfsd]
    *pdpt = 000000002ba33001 *pde = 0000000000000000
    Oops: 0000 [#1] SMP
    Modules linked in: loop nfsd auth_rpcgss ipt_MASQUERADE xt_owner xt_multiport ipt_REJECT xt_tcpudp xt_recent xt_conntrack nf_conntrack_ftp xt_limit xt_LOG iptable_nat nf_conntrack_ipv4 nf_defrag_ipv4 nf_nat_ipv4 nf_nat nf_conntrack iptable_filter ip_tables x_tables af_packet pppoe pppox ppp_generic slhc bridge stp llc tun arc4 iwldvm mac80211 coretemp kvm_intel uvcvideo sdhci_pci sdhci mmc_core videobuf2_vmalloc videobuf2_memops usblp videobuf2_core i915 iwlwifi psmouse videodev cfg80211 kvm fbcon bitblit cfbfillrect acpi_cpufreq mperf evdev softcursor font cfbimgblt i2c_algo_bit cfbcopyarea intel_agp intel_gtt drm_kms_helper snd_hda_codec_conexant drm agpgart fb fbdev tpm_tis thinkpad_acpi tpm nvram e1000e rfkill thermal ptp wmi pps_core tpm_bios 8250_pci processor 8250 ac snd_hda_intel snd_hda_codec snd_pcm battery video i2c_i801 snd_page_alloc snd_timer button serial_core i2c_core snd soundcore thermal_sys hwmon aesni_intel ablk_helper cryp
td lrw aes_i586 xts gf128mul cbc fuse nfs lockd sunrpc dm_crypt dm_mod hid_monterey hid_microsoft hid_logitech hid_ezkey hid_cypress hid_chicony hid_cherry hid_belkin hid_apple hid_a4tech hid_generic usbhid hid sr_mod cdrom sg [last unloaded: microcode]
    Pid: 6374, comm: nfsd Not tainted 3.9.1 #6 LENOVO 4180F65/4180F65
    EIP: 0060:[<f90a3d91>] EFLAGS: 00010202 CPU: 0
    EIP is at nfsd4_client_tracking_exit+0x11/0x50 [nfsd]
    EAX: 00000000 EBX: fffffffe ECX: 00000007 EDX: 00000007
    ESI: eb9dcb00 EDI: eb2991c0 EBP: eb2bde38 ESP: eb2bde34
    DS: 007b ES: 007b FS: 00d8 GS: 00e0 SS: 0068
    CR0: 80050033 CR2: 000003c8 CR3: 2ba80000 CR4: 000407f0
    DR0: 00000000 DR1: 00000000 DR2: 00000000 DR3: 00000000
    DR6: ffff0ff0 DR7: 00000400
    Process nfsd (pid: 6374, ti=eb2bc000 task=eb2711c0 task.ti=eb2bc000)
    Stack:
    fffffffe eb2bde4c f90a3e0c f90a7754 fffffffe eb0a9c00 eb2bdea0 f90a41ed
    eb2991c0 1b270000 eb2991c0 eb2bde7c f9099ce9 eb2bde98 0129a020 eb29a020
    eb2bdecc eb2991c0 eb2bdea8 f9099da5 00000000 eb9dcb00 00000001 67822f08
    Call Trace:
    [<f90a3e0c>] legacy_recdir_name_error+0x3c/0x40 [nfsd]
    [<f90a41ed>] nfsd4_create_clid_dir+0x15d/0x1c0 [nfsd]
    [<f9099ce9>] ? nfsd4_lookup_stateid+0x99/0xd0 [nfsd]
    [<f9099da5>] ? nfs4_preprocess_seqid_op+0x85/0x100 [nfsd]
    [<f90a4287>] nfsd4_client_record_create+0x37/0x50 [nfsd]
    [<f909d6ce>] nfsd4_open_confirm+0xfe/0x130 [nfsd]
    [<f90980b1>] ? nfsd4_encode_operation+0x61/0x90 [nfsd]
    [<f909d5d0>] ? nfsd4_free_stateid+0xc0/0xc0 [nfsd]
    [<f908fd0b>] nfsd4_proc_compound+0x41b/0x530 [nfsd]
    [<f9081b7b>] nfsd_dispatch+0x8b/0x1a0 [nfsd]
    [<f857b85d>] svc_process+0x3dd/0x640 [sunrpc]
    [<f908165d>] nfsd+0xad/0x110 [nfsd]
    [<f90815b0>] ? nfsd_destroy+0x70/0x70 [nfsd]
    [<c1054824>] kthread+0x94/0xa0
    [<c1486937>] ret_from_kernel_thread+0x1b/0x28
    [<c1054790>] ? flush_kthread_work+0xd0/0xd0
    Code: 86 b0 00 00 00 90 c5 0a f9 c7 04 24 70 76 0a f9 e8 74 a9 3d c8 eb ba 8d 76 00 55 89 e5 53 66 66 66 66 90 8b 15 68 c7 0a f9 85 d2 <8b> 88 c8 03 00 00 74 2c 3b 11 77 28 8b 5c 91 08 85 db 74 22 8b
    EIP: [<f90a3d91>] nfsd4_client_tracking_exit+0x11/0x50 [nfsd] SS:ESP 0068:eb2bde34
    CR2: 00000000000003c8
    ---[ end trace 09e54015d145c9c6 ]---

The problem appears to be a regression that was introduced in commit
9a9c6478 "nfsd: make NFSv4 recovery client tracking options per net".
Prior to that commit, it was safe to pass a NULL net pointer to
nfsd4_client_tracking_exit in the legacy recdir case, and
legacy_recdir_name_error did so. After that comit, the net pointer must
be valid.

This patch just fixes legacy_recdir_name_error to pass in a valid net
pointer to that function.

Cc: <stable@vger.kernel.org> # v3.8+
Cc: Stanislav Kinsbursky <skinsbursky@parallels.com>
Reported-and-tested-by: Toralf Förster <toralf.foerster@gmx.de>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>