Bluetooth: Fix locking in bt_accept_dequeue after disconnection

Bluetooth: Fix locking in bt_accept_dequeue after disconnection

Fix a crash that may happen when bt_accept_dequeue is run after a
Bluetooth connection has been disconnected. bt_accept_unlink was called
after release_sock, permitting bt_accept_unlink to run twice on the same
socket and cause a NULL pointer dereference.

[50510.241632] BUG: unable to handle kernel NULL pointer dereference at 00000000000001a8
[50510.241694] IP: [<ffffffffc01243f7>] bt_accept_unlink+0x47/0xa0 [bluetooth]
[50510.241759] PGD 0
[50510.241776] Oops: 0002 [#1] SMP
[50510.241802] Modules linked in: rtl8192cu rtl_usb rtlwifi rtl8192c_common 8021q garp stp mrp llc rfcomm bnep nls_iso8859_1 intel_rapl x86_pkg_temp_thermal intel_powerclamp coretemp arc4 ath9k ath9k_common ath9k_hw ath kvm eeepc_wmi asus_wmi mac80211 snd_hda_codec_hdmi snd_hda_codec_realtek sparse_keymap crct10dif_pclmul snd_hda_codec_generic crc32_pclmul snd_hda_intel snd_hda_controller cfg80211 snd_hda_codec i915 snd_hwdep snd_pcm ghash_clmulni_intel snd_timer snd soundcore serio_raw cryptd drm_kms_helper drm i2c_algo_bit shpchp ath3k mei_me lpc_ich btusb bluetooth 6lowpan_iphc mei lp parport wmi video mac_hid psmouse ahci libahci r8169 mii
[50510.242279] CPU: 0 PID: 934 Comm: krfcommd Not tainted 3.16.0-49-generic #65~14.04.1-Ubuntu
[50510.242327] Hardware name: ASUSTeK Computer INC. VM40B/VM40B, BIOS 1501 12/09/2014
[50510.242370] task: ffff8800d9068a30 ti: ffff8800d7a54000 task.ti: ffff8800d7a54000
[50510.242413] RIP: 0010:[<ffffffffc01243f7>]  [<ffffffffc01243f7>] bt_accept_unlink+0x47/0xa0 [bluetooth]
[50510.242480] RSP: 0018:ffff8800d7a57d58  EFLAGS: 00010246
[50510.242511] RAX: 0000000000000000 RBX: ffff880119bb8c00 RCX: ffff880119bb8eb0
[50510.242552] RDX: ffff880119bb8eb0 RSI: 00000000fffffe01 RDI: ffff880119bb8c00
[50510.242592] RBP: ffff8800d7a57d60 R08: 0000000000000283 R09: 0000000000000001
[50510.242633] R10: 0000000000000000 R11: 0000000000000000 R12: ffff8800d8da9eb0
[50510.242673] R13: ffff8800d74fdb80 R14: ffff880119bb8c00 R15: ffff8800d8da9c00
[50510.242715] FS:  0000000000000000(0000) GS:ffff88011fa00000(0000) knlGS:0000000000000000
[50510.242761] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[50510.242794] CR2: 00000000000001a8 CR3: 0000000001c13000 CR4: 00000000001407f0
[50510.242835] Stack:
[50510.242849]  ffff880119bb8eb0 ffff8800d7a57da0 ffffffffc0124506 ffff8800d8da9eb0
[50510.242899]  ffff8800d8da9c00 ffff8800d9068a30 0000000000000000 ffff8800d74fdb80
[50510.242949]  ffff8800d6f85208 ffff8800d7a57e08 ffffffffc0159985 000000000000001f
[50510.242999] Call Trace:
[50510.243027]  [<ffffffffc0124506>] bt_accept_dequeue+0xb6/0x180 [bluetooth]
[50510.243085]  [<ffffffffc0159985>] l2cap_sock_accept+0x125/0x220 [bluetooth]
[50510.243128]  [<ffffffff810a1b30>] ? wake_up_state+0x20/0x20
[50510.243163]  [<ffffffff8164946e>] kernel_accept+0x4e/0xa0
[50510.243200]  [<ffffffffc05b97cd>] rfcomm_run+0x1ad/0x890 [rfcomm]
[50510.243238]  [<ffffffffc05b9620>] ? rfcomm_process_rx+0x8a0/0x8a0 [rfcomm]
[50510.243281]  [<ffffffff81091572>] kthread+0xd2/0xf0
[50510.243312]  [<ffffffff810914a0>] ? kthread_create_on_node+0x1c0/0x1c0
[50510.243353]  [<ffffffff8176e9d8>] ret_from_fork+0x58/0x90
[50510.243387]  [<ffffffff810914a0>] ? kthread_create_on_node+0x1c0/0x1c0
[50510.243424] Code: 00 48 8b 93 b8 02 00 00 48 8d 83 b0 02 00 00 48 89 51 08 48 89 0a 48 89 83 b0 02 00 00 48 89 83 b8 02 00 00 48 8b 83 c0 02 00 00 <66> 83 a8 a8 01 00 00 01 48 c7 83 c0 02 00 00 00 00 00 00 f0 ff
[50510.243685] RIP  [<ffffffffc01243f7>] bt_accept_unlink+0x47/0xa0 [bluetooth]
[50510.243737]  RSP <ffff8800d7a57d58>
[50510.243758] CR2: 00000000000001a8
[50510.249457] ---[ end trace bb984f932c4e3ab3 ]---

Signed-off-by: Yichen Zhao <zhaoyichen@google.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>