ceph: remove unnecessary assignment in ceph_pre_init_acls()

ceph_pagelist_encode_string() will not fail in reserved case,
also, we do not check err code here, so remove unnecessary
assignment.

Signed-off-by: Chengguang Xu <cgxu519@gmx.com>
Reviewed-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>