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

jira VULN-155003
cve CVE-2023-53297
commit-author Min Li <lm0963hack@gmail.com>
commit 25e97f7

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>
(cherry picked from commit 25e97f7)
	Signed-off-by: Shreeya Patel <spatel@ciq.com>

Author: shreeya-patel98

net/bluetooth/l2cap_core.c

@@ -4682,7 +4682,6 @@ static inline int l2cap_disconnect_rsp(struct l2cap_conn *conn,
 
 	chan = __l2cap_get_chan_by_scid(conn, scid);
 	if (!chan) {
-		mutex_unlock(&conn->chan_lock);
 		return 0;
 	}