gfs2: check context in gfs2_glock_put

Add a might_sleep call into gfs2_glock_put which can sleep in DLM when
the last reference is released.  This will show problems earlier, and
not only when the last reference is put.
Signed-off-by: Alexander Aring <aahringo@redhat.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>

fs/gfs2/glock.c

@@ -301,6 +301,9 @@ void gfs2_glock_queue_put(struct gfs2_glock *gl)
 
 void gfs2_glock_put(struct gfs2_glock *gl)
 {
+	/* last put could call sleepable dlm api */
+	might_sleep();
+
 	if (lockref_put_or_lock(&gl->gl_lockref))
 		return;