cramfs: fix usage on non-MTD device

When both CONFIG_CRAMFS_MTD and CONFIG_CRAMFS_BLOCKDEV are enabled, if
we fail to mount on MTD, we don't try on block device.

Note: this relies upon cramfs_mtd_fill_super() leaving no side
effects on fc state in case of failure; in general, failing
get_tree_...() does *not* mean "fine to try again"; e.g. parsed
options might've been consumed by fill_super callback and freed
on failure.

Fixes: 74f78fc5 ("vfs: Convert cramfs to use the new mount API")
Signed-off-by: Maxime Bizon <mbizon@freebox.fr>
Signed-off-by: Nicolas Pitre <nico@fluxnic.net>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/cramfs/inode.c

@@ -958,8 +958,8 @@ static int cramfs_get_tree(struct fs_context *fc)
 
 	if (IS_ENABLED(CONFIG_CRAMFS_MTD)) {
 		ret = get_tree_mtd(fc, cramfs_mtd_fill_super);
-		if (ret < 0)
-			return ret;
+		if (!ret)
+			return 0;
 	}
 	if (IS_ENABLED(CONFIG_CRAMFS_BLOCKDEV))
 		ret = get_tree_bdev(fc, cramfs_blkdev_fill_super);