ext4: reset error code in ext4_find_entry in fallback

When ext4_find_entry() falls back to "searching the old fashioned
way" due to a corrupt dx dir, it needs to reset the error code
to NULL so that the nonstandard ERR_BAD_DX_DIR code isn't returned
to userspace.

https://bugzilla.kernel.org/show_bug.cgi?id=199947

Reported-by: Anatoly Trosinenko <anatoly.trosinenko@yandex.com>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@vger.kernel.org

fs/ext4/namei.c

@@ -1398,6 +1398,7 @@ static struct buffer_head * ext4_find_entry (struct inode *dir,
 			goto cleanup_and_exit;
 		dxtrace(printk(KERN_DEBUG "ext4_find_entry: dx failed, "
 			       "falling back\n"));
+		ret = NULL;
 	}
 	nblocks = dir->i_size >> EXT4_BLOCK_SIZE_BITS(sb);
 	if (!nblocks) {