ix(4) handles 2048 byte rx buffers, so consistently use 2048 everywhere

... except when we allocate those buffers with some ETHER_ALIGN extra
bytes so we can properly align the packet payloads.

this is the same as if_igc.c r1.27.

ok?

Index: if_ix.c
===================================================================
RCS file: /cvs/src/sys/dev/pci/if_ix.c,v
diff -u -p -r1.217 if_ix.c
--- if_ix.c	4 Sep 2024 07:54:52 -0000	1.217
+++ if_ix.c	6 Sep 2024 23:03:14 -0000
@@ -764,7 +764,7 @@ ixgbe_init(void *arg)
 	ixgbe_initialize_transmit_units(sc);
 
 	/* Use 2k clusters, even for jumbo frames */
-	sc->rx_mbuf_sz = MCLBYTES + ETHER_ALIGN;
+	sc->rx_mbuf_sz = MCLBYTES;
 
 	/* Prepare receive descriptors and buffers */
 	if (ixgbe_setup_receive_structures(sc)) {
@@ -2697,12 +2694,11 @@ ixgbe_get_buf(struct ix_rxring *rxr, int
 		return (ENOBUFS);
 	}
 
-	/* needed in any case so preallocate since this one will fail for sure */
-	mp = MCLGETL(NULL, M_DONTWAIT, sc->rx_mbuf_sz);
+	mp = MCLGETL(NULL, M_DONTWAIT, sc->rx_mbuf_sz + ETHER_ALIGN);
 	if (!mp)
 		return (ENOBUFS);
 
-	mp->m_data += (mp->m_ext.ext_size - sc->rx_mbuf_sz);
+	mp->m_data += ETHER_ALIGN;
 	mp->m_len = mp->m_pkthdr.len = sc->rx_mbuf_sz;
 
 	error = bus_dmamap_load_mbuf(rxr->rxdma.dma_tag, rxbuf->map,
@@ -2747,8 +2743,9 @@ ixgbe_allocate_receive_buffers(struct ix
 
 	rxbuf = rxr->rx_buffers;
 	for (i = 0; i < sc->num_rx_desc; i++, rxbuf++) {
-		error = bus_dmamap_create(rxr->rxdma.dma_tag, 16 * 1024, 1,
-		    16 * 1024, 0, BUS_DMA_NOWAIT, &rxbuf->map);
+		error = bus_dmamap_create(rxr->rxdma.dma_tag,
+		    sc->rx_mbuf_sz, 1, sc->rx_mbuf_sz, 0,
+		    BUS_DMA_NOWAIT, &rxbuf->map);
 		if (error) {
 			printf("%s: Unable to create Pack DMA map\n",
 			    ifp->if_xname);
@@ -2789,7 +2786,8 @@ ixgbe_setup_receive_ring(struct ix_rxrin
 	rxr->next_to_check = 0;
 	rxr->last_desc_filled = sc->num_rx_desc - 1;
 
-	if_rxr_init(&rxr->rx_ring, 2 * ((ifp->if_hardmtu / MCLBYTES) + 1),
+	if_rxr_init(&rxr->rx_ring,
+	    2 * howmany(ifp->if_hardmtu, MCLBYTES) + 1,
 	    sc->num_rx_desc - 1);
 
 	ixgbe_rxfill(rxr);
@@ -2924,7 +2922,7 @@ ixgbe_initialize_receive_units(struct ix
 		IXGBE_WRITE_REG(hw, IXGBE_RDRXCTL, rdrxctl);
 	}
 
-	bufsz = (sc->rx_mbuf_sz - ETHER_ALIGN) >> IXGBE_SRRCTL_BSIZEPKT_SHIFT;
+	bufsz = sc->rx_mbuf_sz >> IXGBE_SRRCTL_BSIZEPKT_SHIFT;
 
 	for (i = 0; i < sc->num_queues; i++, rxr++) {
 		uint64_t rdba = rxr->rxdma.dma_map->dm_segs[0].ds_addr;