ceph: fix the incorrect comment for the ceph_mds_caps struct

The incorrect comment is misleading. Acutally the last members
in ceph_mds_caps strcut is a union for none export and export
bodies.

Signed-off-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>