ifconfig does not honor disabling vlanhwtag

[vmaffione]

Summary

The FreeBSD em driver fails to properly reset the VME flag in the e1000 CTRL register during the following ifconfig command

ifconfig em1 -vlanhwtag

at least on the e1000 device as emulated by QEMU

https://github.com/luigirizzo/netmap/issues/703
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=236584

[vmaffione]

Any opinions about this?

head/sys/dev/e1000/if_em.c

 #ifdef INVARIANTS
                 toep->ulpcb2 = NULL;
 #endif
                 return (0);
         }
 
         tp = intotcpcb(inp);
         MPASS(icp->icp_seq == tp->rcv_nxt);
-        MPASS(tp->rcv_wnd >= pdu_len);
         tp->rcv_nxt += pdu_len;
-        tp->rcv_wnd -= pdu_len;
         tp->t_rcvtime = ticks;
 
-        /* update rx credits */
-        t4_rcvd(&toep->td->tod, tp);        /* XXX: sc->tom_softc.tod */
+        /*
+         * Don't update the window size or return credits since RX
+         * flow control is disabled.
+         */
 
         so = inp->inp_socket;
         sb = &so->so_rcv;
         SOCKBUF_LOCK(sb);
 
         icc = toep->ulpcb;
         if (__predict_false(icc == NULL || sb->sb_state & SBS_CANTRCVMORE)) {
                 CTR5(KTR_CXGBE,