fs: btrfs: initialize @ret to 0 to prevent uninitialized return value

In show_dir() if we hit a ROOT_ITEM, we can exit with uninitialized
@ret.

Fix it by initializing it to 0.

Reported-by: Coverity CID 312955
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Marek Behún <marek.behun@nic.cz>

fs/btrfs/btrfs.c

@@ -36,7 +36,7 @@ static int show_dir(struct btrfs_root *root, struct extent_buffer *eb,
 	char *target = NULL;
 	char filetime[32];
 	time_t mtime;
-	int ret;
+	int ret = 0;
 
 	btrfs_dir_item_key_to_cpu(eb, di, &key);