fix.diff - DF-4270: handle wb_encap() failure in wb_start().

wb_start() ignored wb_encap()'s return; the two coalesce-failure paths
(MGETHDR/MCLGET under M_NOWAIT) leak the already-dequeued packet and leave a
half-programmed, unterminated fragment chain that the NIC can be started on.

Applies on top of sys/dev/netif/wb/if_wb.c (read-only audit tree; never applied).

--- a/sys/dev/netif/wb/if_wb.c
+++ b/sys/dev/netif/wb/if_wb.c
@@ -1322,11 +1322,32 @@ wb_start(struct ifnet *ifp, struct ifaltq_subque *ifsq)
 		sc->wb_cdata.wb_tx_free = cur_tx->wb_nextdesc;

 		/* Pack the data into the descriptor. */
-		wb_encap(sc, cur_tx, m_head);
+		if (wb_encap(sc, cur_tx, m_head) != 0) {
+			int fidx;
+
+			/*
+			 * wb_encap() failed (mbuf shortage): free the
+			 * dequeued packet, disarm the partially programmed
+			 * fragments so the NIC never walks the unterminated
+			 * chain, and put the descriptor back on the free
+			 * list.
+			 */
+			m_freem(m_head);
+			cur_tx->wb_mbuf = NULL;
+			for (fidx = 0; fidx < WB_MAXFRAGS; fidx++)
+				cur_tx->wb_ptr->wb_frag[fidx].wb_status = 0;
+			sc->wb_cdata.wb_tx_free = cur_tx;
+			if (cur_tx == start_tx)
+				return;		/* nothing was queued */
+			break;
+		}

 		if (cur_tx != start_tx)
 			WB_TXOWN(cur_tx) = WB_TXSTAT_OWN;