bluetooth l2cap: fix locking in l2cap_global_chan_by_psm

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

commit 23691d75cdc69c3b285211b4d77746aa20a17d18
Author: Gustavo F. Padovan <padovan@profusion.mobi>
Date:   Wed Apr 27 18:26:32 2011 -0300

    Bluetooth: Remove l2cap_sk_list

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c
index a86f9ba..e64a1c2 100644
--- a/net/bluetooth/l2cap_core.c
+++ b/net/bluetooth/l2cap_core.c
@@ -906,7 +906,7 @@
 		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;
 			}