vtnet: Better adjust for ethernet alignment.

Move adjustment of the mbuf from where we allocate it to where we are
about to queue it to the device. Do this only on those platforms that
require it. This allows us to receive an entire jumbo frame on other
platforms. It also doesn't make the adjustment on subsequent frames when
we queue mulitple mbufs for LRO operations.

For the normal use case on armv7, there's no difference because we only
ever allocate one mbuf. However, for the LRO cases it increases what's
available in LRO. It also ensure that we get enough mbufs in those cases
as well (though I have no ability to test this on a LRO scenario with
armv7).

This has the side effect of reverting 527b62e37e68.

Fixes:			527b62e37e68
Sponsored by:		Netflix

sys/dev/virtio/network/if_vtnet.c

@@ -1532,8 +1532,8 @@ vtnet_rx_alloc_buf(struct vtnet_softc *sc, int nbufs, struct mbuf **m_tailp)
 			m_freem(m_head);
 			return (NULL);
 		}
+
 		m->m_len = size;
-		m_adj(m, ETHER_ALIGN);
 		if (m_head != NULL) {
 			m_tail->m_next = m;
 			m_tail = m;
@@ -1587,6 +1587,15 @@ vtnet_rxq_replace_lro_nomrg_buf(struct vtnet_rxq *rxq, struct mbuf *m0,
 		    ("%s: mbuf size %d not expected cluster size %d", __func__,
 		    m->m_len, clustersz));
 
+#ifdef __NO_STRICT_ALIGNMENT
+		/*
+		 * Need to offset the first mbuf in this chain to align the IP
+		 * structure because this host requires strict alignment.
+		 */
+		if (m_prev == NULL) {
+			m_adj(m, ETHER_ALIGN);
+		}
+#endif
 		m->m_len = MIN(m->m_len, len);
 		len -= m->m_len;
 
@@ -1655,6 +1664,13 @@ vtnet_rxq_replace_buf(struct vtnet_rxq *rxq, struct mbuf *m, int len)
 	if (m_new == NULL)
 		return (ENOBUFS);
 
+#ifdef __NO_STRICT_ALIGNMENT
+	/*
+	 * Need to offset the first mbuf in this chain to align the IP
+	 * structure because this host requires strict alignment.
+	 */
+	m_adj(m_new, ETHER_ALIGN);
+#endif
 	error = vtnet_rxq_enqueue_buf(rxq, m_new);
 	if (error) {
 		sc->vtnet_stats.rx_enq_replacement_failed++;
@@ -1722,6 +1738,13 @@ vtnet_rxq_new_buf(struct vtnet_rxq *rxq)
 	if (m == NULL)
 		return (ENOBUFS);
 
+#ifdef __NO_STRICT_ALIGNMENT
+	/*
+	 * Need to offset the first mbuf in this chain to align the IP
+	 * structure because this host requires strict alignment.
+	 */
+	m_adj(m, ETHER_ALIGN);
+#endif
 	error = vtnet_rxq_enqueue_buf(rxq, m);
 	if (error)
 		m_freem(m);
@@ -1907,8 +1930,10 @@ vtnet_rxq_discard_buf(struct vtnet_rxq *rxq, struct mbuf *m)
 	int error __diagused;
 
 	/*
-	 * Requeue the discarded mbuf. This should always be successful
-	 * since it was just dequeued.
+	 * Requeue the discarded mbuf. This should always be successful since it
+	 * was just dequeued. There's no need to adjust for ethernet alignment
+	 * here on strict alignment hosts because we're requeueing a packet
+	 * already adjusted.
 	 */
 	error = vtnet_rxq_enqueue_buf(rxq, m);
 	KASSERT(error == 0,