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: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>