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

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>