Btrfs: Handle transid == 0 while opening devices

Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 501d23d..3fbf083 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -262,7 +262,7 @@
 			goto error_brelse;
 
 		transid = btrfs_super_generation(disk_super);
-		if (transid > latest_transid) {
+		if (!latest_transid || transid > latest_transid) {
 			latest_devid = devid;
 			latest_transid = transid;
 			latest_bdev = bdev;