ceph_spg_compare
int ceph_spg_compare(const struct ceph_spg *lhs, const struct ceph_spg *rhs);
DEFINE_RB_FUNCS2(spg_mapping, struct ceph_spg_mapping, spgid, ceph_spg_compare,
if (!ceph_spg_compare(&req->r_t.spgid, &m->spgid)) {