bluetooth l2cap: fix locking in l2cap_global_chan_by_psm

read_lock() ... read_unlock_bh() is clearly bogus.
This was broken by

commit 23691d75
Author: Gustavo F. Padovan <padovan@profusion.mobi>
Date:   Wed Apr 27 18:26:32 2011 -0300

    Bluetooth: Remove l2cap_sk_list
Signed-off-by: NJohannes Berg <johannes.berg@intel.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

net/bluetooth/l2cap_core.c

@@ -906,7 +906,7 @@ static struct l2cap_chan *l2cap_global_chan_by_psm(int state, __le16 psm, bdaddr
 		if (c->psm == psm) {
 			/* Exact match. */
 			if (!bacmp(&bt_sk(sk)->src, src)) {
-				read_unlock_bh(&chan_list_lock);
+				read_unlock(&chan_list_lock);
 				return c;
 			}