CEPH_BANNER_V2_PREFIX_LEN
buf = alloc_conn_buf(con, CEPH_BANNER_V2_PREFIX_LEN);
add_in_kvec(con, buf, CEPH_BANNER_V2_PREFIX_LEN);
add_in_sign_kvec(con, buf, CEPH_BANNER_V2_PREFIX_LEN);
WARN_ON(con->v2.in_kvecs[0].iov_len != CEPH_BANNER_V2_PREFIX_LEN);