dm snapshot: fix metadata writing when suspending

When suspending a device-mapper device, dm_suspend() sleeps until all
necessary I/O is completed.  This state is triggered by a callback from
persistent_commit().  But some I/O can still be issued *after* the callback
(to prepare the next metadata area for use if the current one is full).  This
patch delays the callback until after that I/O is complete.
Signed-off-by: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Signed-off-by: Adrian Bunk <bunk@stusta.de>

drivers/md/dm-exception-store.c

@@ -517,14 +517,6 @@ static void persistent_commit(struct exception_store *store,
 		if (r)
 			ps->valid = 0;
 
-		for (i = 0; i < ps->callback_count; i++) {
-			cb = ps->callbacks + i;
-			cb->callback(cb->context, r == 0 ? 1 : 0);
-		}
-
-		ps->callback_count = 0;
-	}
-
 		/*
 		 * Have we completely filled the current area ?
 		 */
@@ -534,6 +526,14 @@ static void persistent_commit(struct exception_store *store,
 			if (r)
 				ps->valid = 0;
 		}
+
+		for (i = 0; i < ps->callback_count; i++) {
+			cb = ps->callbacks + i;
+			cb->callback(cb->context, r == 0 ? 1 : 0);
+		}
+
+		ps->callback_count = 0;
+	}
 }
 
 static void persistent_drop(struct exception_store *store)