Btrfs: make subvolid=0 mount the original default root

Since theres not a good way to make sure the user sees the original default root
tree id, and not to mention it's 5 so is way different than any other volume,
just make subvol=0 mount the original default root.  This makes it a bit easier
for users to handle in the long run.  Thanks,
Signed-off-by: NJosef Bacik <josef@redhat.com>
Signed-off-by: NChris Mason <chris.mason@oracle.com>

fs/btrfs/super.c

@@ -325,9 +325,15 @@ static int btrfs_parse_early_options(const char *options, fmode_t flags,
 			break;
 		case Opt_subvolid:
 			intarg = 0;
-			match_int(&args[0], &intarg);
-			if (intarg)
-				*subvol_objectid = intarg;
+			error = match_int(&args[0], &intarg);
+			if (!error) {
+				/* we want the original fs_tree */
+				if (!intarg)
+					*subvol_objectid =
+						BTRFS_FS_TREE_OBJECTID;
+				else
+					*subvol_objectid = intarg;
+			}
 			break;
 		case Opt_device:
 			error = btrfs_scan_one_device(match_strdup(&args[0]),