btrfs: fix uninitialized ret in ref-verify

commit c5f4987e86f6692fdb12533ea1fc7a7bb98e555a upstream.

Coverity caught a case where we could return with a uninitialized value
in ret in process_leaf.  This is actually pretty likely because we could
very easily run into a block group item key and have a garbage value in
ret and think there was an errror.  Fix this by initializing ret to 0.

Reported-by: Colin Ian King <colin.king@canonical.com>
Fixes: fd708b81 ("Btrfs: add a extent ref verify tool")
CC: stable@vger.kernel.org # 4.19+
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>