dm: add missing memory barrier to dm_suspend

Add memory barrier to fix atomic_read of pending value.
Signed-off-by: Milan Broz <mbroz@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

drivers/md/dm.c

@@ -1410,6 +1410,7 @@ int dm_suspend(struct mapped_device *md, unsigned suspend_flags)
 	while (1) {
 		set_current_state(TASK_INTERRUPTIBLE);
 
+		smp_mb();
 		if (!atomic_read(&md->pending) || signal_pending(current))
 			break;