Fix to a panic found internally, bad pointer during rxeof

processing. Thanks for John Baldwin for catching this. Not
clearing the flag member of the rxbuf could result in a NULL
mbuf pointer being used.

MFC after:	2 days (this needs to get into 9.2!)

sys/dev/ixgbe/ixgbe.c

@@ -3943,6 +3943,7 @@ ixgbe_free_receive_ring(struct rx_ring *rxr)
 			rxbuf->buf->m_flags |= M_PKTHDR;
 			m_freem(rxbuf->buf);
 			rxbuf->buf = NULL;
+			rxbuf->flag = 0;
 		}
 	}
 }