gfs2: using posix_acl_xattr_size instead of posix_acl_to_xattr

It seems better to get size by calling posix_acl_xattr_size() instead of
calling posix_acl_to_xattr() with NULL buffer argument.

posix_acl_xattr_size() never returns 0, so remove the unnecessary check.

Signed-off-by: Chengguang Xu <cgxu519@gmx.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>