ovl: do not copy attr several times

In ovl_xattr_set() we have already copied attr of real inode
so no need to copy it again in ovl_posix_acl_xattr_set().

Signed-off-by: Chengguang Xu <cgxu519@mykernel.net>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>