Bluetooth: L2CAP: fix "bad unlock balance" in l2cap_disconnect_rsp

[ Upstream commit 25e97f7b1866e6b8503be349eeea44bb52d661ce ]

conn->chan_lock isn't acquired before l2cap_get_chan_by_scid,
if l2cap_get_chan_by_scid returns NULL, then 'bad unlock balance'
is triggered.

Reported-by:  <syzbot+9519d6b5b79cf7787cf3@syzkaller.appspotmail.com>
Link: https://lore.kernel.org/all/000000000000894f5f05f95e9f4d@google.com/


Signed-off-by: Min Li <lm0963hack@gmail.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>