gfs2: Add missing rcu locking for glock lookup

gfs2: Add missing rcu locking for glock lookup

BugLink: http://bugs.launchpad.net/bugs/1672544
commit f38e5fb95a1f8feda88531eedc98f69b24748712 upstream.

We must hold the rcu read lock across looking up glocks and trying to
bump their refcount to prevent the glocks from being freed in between.

Signed-off-by: Andrew Price <anprice@redhat.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>