ceph: rename struct ceph_acls_info to ceph_acl_sec_ctx

Also rename ceph_release_acls_info() to ceph_release_acl_sec_ctx().
And move their definitions to different files. This is preparation
for security label support.

Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>