Btrfs: fix check_path_shared so it returns the right value

When running xfstests 224 I kept getting ENOSPC when trying to remove the files,
and this is because we were returning ret from check_path_shared while it was
uninitalized, which isn't right.  Fix this to return 0 properly, and now
xfstests 224 doesn't freak out when it tries to clean itself up.  Thanks,

Signed-off-by: Josef Bacik <josef@redhat.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>