Bluetooth: Fix locking issue on SCO disconnection

Bluetooth: Fix locking issue on SCO disconnection

Thread handling SCO disconnection may get preempted in '__sco_sock_close'
after dropping a reference to hci_conn but before marking this as NULL
in associated struct sco_conn. When execution returs to this thread,
this connection will possibly be released, resulting in kernel crash

Lock connection before this point.

BUG: unable to handle kernel NULL pointer dereference at   (null)
IP: [<fb770ab9>] __sco_sock_close+0x194/0x1ff [bluetooth]
*pdpt = 0000000023da6001 *pde = 0000000000000000
Oops: 0002 [#1] PREEMPT SMP
Modules linked in: evdev ecb rfcomm(O) libcomposite usb2380 udc_core bnep(O) btusb(O) btbcm(O) cdc_acm btintel(O) bluetooth(O) arc4 uinput hid_multitouch usbhid iwlmvm(O) hide
Pid: 984, comm: bluetooth Tainted: G           O 3.8.0-115.1-plk-adaptation-byt-ivi-brd #1
EIP: 0060:[<fb770ab9>] EFLAGS: 00010282 CPU: 2
EIP is at __sco_sock_close+0x194/0x1ff [bluetooth]
EAX: 00000000 EBX: e49d7600 ECX: ef1ec3c2 EDX: 000000c3
ESI: e4c12000 EDI: 00000000 EBP: ef1edf5c ESP: ef1edf4c
 DS: 007b ES: 007b FS: 00d8 GS: 0033 SS: 0068
CR0: 80050033 CR2: 00000000 CR3: 23da7000 CR4: 001007f0
DR0: 00000000 DR1: 00000000 DR2: 00000000 DR3: 00000000
DR6: ffff0ff0 DR7: 00000400
Process bluetooth (pid: 984, ti=ef1ec000 task=e47f2550 task.ti=ef1ec000)
Stack:
 e4c120d0 e49d7600 00000000 08421a40 ef1edf70 fb770b7a 00000002 e8a4cc80
 08421a40 ef1ec000 c12966b1 00000001 00000000 0000000b 084954c8 c1296b6c
 0000001b 00000002 0000001b 00000002 00000000 00000002 b2524880 00000046
Call Trace:
 [<fb770b7a>] ? sco_sock_shutdown+0x56/0x95 [bluetooth]
 [<c12966b1>] ? sys_shutdown+0x37/0x53
 [<c1296b6c>] ? sys_socketcall+0x12e/0x1be
 [<c134ae7e>] ? sysenter_do_call+0x12/0x26
 [<c1340000>] ? ip_vs_control_net_cleanup+0x46/0xb1
Code: e8 90 6b 8c c5 f6 05 72 5d 78 fb 04 74 17 8b 46 08 50 56 68 0a fd 77 fb 68 60 5d 78 fb e8 68 95 9e c5 83 c4 10 8b 83 fc 01 00 00 <c7> 00 00 00 00 00 eb 32 ba 68 00 00 0b
EIP: [<fb770ab9>] __sco_sock_close+0x194/0x1ff [bluetooth] SS:ESP 0068:ef1edf4c
CR2: 0000000000000000
---[ end trace 47fa2f55a9544e69 ]---

Signed-off-by: Kuba Pawlak <kubax.t.pawlak@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>