fix vmxnet3 crash when LRO is enabled

The crash can occur when all of the following conditions are true:
- a packet consists of multiple segements (requires LRO enabled)
- there has been a failure to allocate an mbuf for the packet and
  the packet has to be dropped
- a host (vmware) still owned at least one segment of the packet,
  so the driver had to wait for another interrupt to proceed to
  discarding the remaning segment(s)

Reviewed by:	rstone
MFC after:	2 weeks
Sponsored by:	Panzura
Differential Revision: https://reviews.freebsd.org/D10874

sys/dev/vmware/vmxnet3/if_vmx.c

@@ -2194,6 +2194,20 @@ vmxnet3_rxq_eof(struct vmxnet3_rxqueue *rxq)
 		} else {
 			KASSERT(rxd->btype == VMXNET3_BTYPE_BODY,
 			    ("%s: non start of frame w/o body buffer", __func__));
+
+			if (m_head == NULL && m_tail == NULL) {
+				/*
+				 * This is a continuation of a packet that we
+				 * started to drop, but could not drop entirely
+				 * because this segment was still owned by the
+				 * host.  So, drop the remainder now.
+				 */
+				vmxnet3_rxq_eof_discard(rxq, rxr, idx);
+				if (!rxcd->eop)
+					vmxnet3_rxq_discard_chain(rxq);
+				goto nextp;
+			}
+
 			KASSERT(m_head != NULL,
 			    ("%s: frame not started?", __func__));