Bluetooth: prefetch channel before killing sock

[ Upstream commit 2a154903cec20fb64ff4d7d617ca53c16f8fd53a ]

Prefetch channel before killing sock in order to fix UAF like

 BUG: KASAN: use-after-free in l2cap_sock_release+0x24c/0x290 net/bluetooth/l2cap_sock.c:1212
 Read of size 8 at addr ffff8880944904a0 by task syz-fuzzer/9751

Reported-by:  <syzbot+c3c5bdea7863886115dc@syzkaller.appspotmail.com>
Fixes: 6c08fc896b60 ("Bluetooth: Fix refcount use-after-free issue")
Cc: Manish Mandlik <mmandlik@google.com>
Signed-off-by: Hillf Danton <hdanton@sina.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>