Bluetooth: hci_conn: Fix possible UAF

This fixes the following trace:

==================================================================
BUG: KASAN: slab-use-after-free in hci_conn_del+0xba/0x3a0
Write of size 8 at addr ffff88800208e9c8 by task iso-tester/31

CPU: 0 PID: 31 Comm: iso-tester Not tainted 6.3.0-rc2-g991aa4a69a47
 #4716
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.1-2.fc36
04/01/2014
Call Trace:
 <TASK>
 dump_stack_lvl+0x1d/0x70
 print_report+0xce/0x610
 ? __virt_addr_valid+0xd4/0x150
 ? hci_conn_del+0xba/0x3a0
 kasan_report+0xdd/0x110
 ? hci_conn_del+0xba/0x3a0
 hci_conn_del+0xba/0x3a0
 hci_conn_hash_flush+0xf2/0x120
 hci_dev_close_sync+0x388/0x920
 hci_unregister_dev+0x122/0x260
 vhci_release+0x4f/0x90
 __fput+0x102/0x430
 task_work_run+0xf1/0x160
 ? __pfx_task_work_run+0x10/0x10
 ? mark_held_locks+0x24/0x90
 exit_to_user_mode_prepare+0x170/0x180
 syscall_exit_to_user_mode+0x19/0x50
 do_syscall_64+0x4e/0x90
 entry_SYSCALL_64_after_hwframe+0x70/0xda

Fixes: 0f00cd322d22 ("Bluetooth: Free potentially unfreed SCO connection")
Link: https://syzkaller.appspot.com/bug?extid=8bb72f86fc823817bc5d
Cc: <stable@vger.kernel.org>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
