ceph: only set CEPH_I_SEC_INITED if we got a MAC label

__ceph_getxattr will set the CEPH_I_SEC_INITED flag whenever it gets
any xattr that starts with "security.". We only want to set that flag
when fetching the MAC label for the currently-active LSM, however.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>