dm snapshot: free exception store on init failure

While initializing the snapshot module, if we fail to register
the snapshot target then we must back-out the exception store
module initialization.

Cc: stable@kernel.org
Signed-off-by: Jonathan Brassow <jbrassow@redhat.com>
Reviewed-by: Mikulas Patocka <mpatocka@redhat.com>
Reviewed-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

drivers/md/dm-snap.c

@@ -1470,7 +1470,7 @@ static int __init dm_snapshot_init(void)
 	r = dm_register_target(&snapshot_target);
 	if (r) {
 		DMERR("snapshot target register failed %d", r);
-		return r;
+		goto bad_register_snapshot_target;
 	}
 
 	r = dm_register_target(&origin_target);
@@ -1527,6 +1527,9 @@ bad2:
 	dm_unregister_target(&origin_target);
 bad1:
 	dm_unregister_target(&snapshot_target);
+
+bad_register_snapshot_target:
+	dm_exception_store_exit();
 	return r;
 }