cxgbe(4): Report accurate rx_buf_maxsize to netmap.

MFC after:	2 weeks
Sponsored by:	Chelsio Communications

sys/dev/cxgbe/t4_netmap.c

@@ -991,6 +991,7 @@ cxgbe_nm_attach(struct vi_info *vi)
 	na.nm_register = cxgbe_netmap_reg;
 	na.num_tx_rings = vi->nnmtxq;
 	na.num_rx_rings = vi->nnmrxq;
+	na.rx_buf_maxsize = MAX_MTU;
 	netmap_attach(&na);	/* This adds IFCAP_NETMAP to if_capabilities */
 }