staging: usbip: vhci: update reference count for usb_device

staging: usbip: vhci: update reference count for usb_device

This fixes an oops observed when reading status during
removal of a device:

[ 1706.648285] general protection fault: 0000 [#1] SMP
[ 1706.648294] last sysfs file: /sys/devices/platform/vhci_hcd/status
[ 1706.648297] CPU 1
[ 1706.648300] Modules linked in: binfmt_misc microcode fuse loop vhci_hcd(N) usbip(N) usbcore usbip_common_mod(N) rtc_core rtc_lib joydev dm_mirror dm_region_hash dm_log linear dm_snapshot xennet dm_mod ext3 mbcache jbd processor thermal_sys hwmon xenblk cdrom
[ 1706.648324] Supported: Yes
[ 1706.648327] Pid: 10422, comm: usbip Tainted: G          N  2.6.32.12-0.7-xen #1
[ 1706.648330] RIP: e030:[<ffffffff801b10d5>]  [<ffffffff801b10d5>] strnlen+0x5/0x40
[ 1706.648340] RSP: e02b:ffff8800a994dd30  EFLAGS: 00010286
[ 1706.648343] RAX: ffffffff80481ec1 RBX: 0000000000000000 RCX: 0000000000000002
[ 1706.648347] RDX: 00200d1d4f1c001c RSI: ffffffffffffffff RDI: 00200d1d4f1c001c
[ 1706.648350] RBP: ffff880129a1c0aa R08: ffffffffa01901c4 R09: 0000000000000006
[ 1706.648353] R10: 0000000000000000 R11: 0000000000000000 R12: ffff8800a9a1c0ab
[ 1706.648357] R13: 00200d1d4f1c001c R14: 00000000ffffffff R15: ffff880129a1c0aa
[ 1706.648363] FS:  00007f2f2e9ca700(0000) GS:ffff880001018000(0000) knlGS:0000000000000000
[ 1706.648367] CS:  e033 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 1706.648370] CR2: 000000000071b048 CR3: 00000000b4b68000 CR4: 0000000000002660
[ 1706.648374] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 1706.648378] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[ 1706.648381] Process usbip (pid: 10422, threadinfo ffff8800a994c000, task ffff88007b170200)
[ 1706.648385] Stack:
[ 1706.648387]  ffffffff801b28c9 0000000000000002 ffffffffa01901c4 ffff8800a9a1c0ab
[ 1706.648391] <0> ffffffffa01901c6 ffff8800a994de08 ffffffff801b339b 0000000000000004
[ 1706.648397] <0> 0000000affffffff ffffffffffffffff 00000000000067c0 0000000000000000
[ 1706.648404] Call Trace:
[ 1706.648413]  [<ffffffff801b28c9>] string+0x39/0xe0
[ 1706.648419]  [<ffffffff801b339b>] vsnprintf+0x1eb/0x620
[ 1706.648423]  [<ffffffff801b3813>] sprintf+0x43/0x50
[ 1706.648429]  [<ffffffffa018d719>] show_status+0x1b9/0x220 [vhci_hcd]
[ 1706.648438]  [<ffffffff8024a2b7>] dev_attr_show+0x27/0x60
[ 1706.648445]  [<ffffffff80144821>] sysfs_read_file+0x101/0x1d0
[ 1706.648451]  [<ffffffff800da4a7>] vfs_read+0xc7/0x130
[ 1706.648457]  [<ffffffff800da613>] sys_read+0x53/0xa0
[ 1706.648462]  [<ffffffff80007458>] system_call_fastpath+0x16/0x1b
[ 1706.648468]  [<00007f2f2de40f30>] 0x7f2f2de40f30
[ 1706.648470] Code: 66 0f 1f 44 00 00 48 83 c2 01 80 3a 00 75 f7 48 89 d0 48 29 f8 f3 c3 66 66 66 66 66 66 2e 0f 1f 84 00 00 00 00 00 48 85 f6 74 29 <80> 3f 00 74 24 48 8d 56 ff 48 89 f8 eb 0e 0f 1f 44 00 00 48 83
[ 1706.648507] RIP  [<ffffffff801b10d5>] strnlen+0x5/0x40
[ 1706.648511]  RSP <ffff8800a994dd30>
[ 1706.649575] ---[ end trace b4eb72bf2e149593 ]---

Signed-off-by: Max Vozeler <max@vozeler.com>
Tested-by: Mark Wehby <MWehby@luxotticaRetail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>