hn: Invoke the owned VF's transmit queue flush callback

The generic if_qflush() only clears if_snd.  Calling it directly for a
transparent VF bypasses the driver's callback and its private software
transmit queues.  Add if_getqflushfn(), matching the existing callback
accessors, and invoke the registered callback while hn_vf_lock protects
the VF pointer.

Flush an attached VF whenever transparent mode owns it, not only while
the VF datapath is enabled.  Disabling transmit during handoff does not
discard packets already queued to the VF.  This also covers the inactive
interval after fallback.  Non-transparent VFs remain independently owned
and are not flushed by hn.  Synthetic queue flushing is unchanged.

MFC after:	2 weeks
Sponsored by:	BBOX.io

sys/dev/hyperv/netvsc/if_hn.c

@@ -6240,8 +6240,9 @@ hn_xmit_qflush(if_t ifp)
 	if_qflush(ifp);
 
 	rm_rlock(&sc->hn_vf_lock, &pt);
-	if (sc->hn_xvf_flags & HN_XVFFLAG_ENABLED)
-		if_qflush(sc->hn_vf_ifp);
+	/* Transparent mode owns the VF's queues even while switching paths. */
+	if (hn_xpnt_vf && sc->hn_vf_ifp != NULL)
+		if_getqflushfn(sc->hn_vf_ifp)(sc->hn_vf_ifp);
 	rm_runlock(&sc->hn_vf_lock, &pt);
 }
 

sys/net/if.c

@@ -5087,6 +5087,12 @@ if_setqflushfn(if_t ifp, if_qflush_fn_t flush_fn)
 	ifp->if_qflush = flush_fn;
 }
 
+if_qflush_fn_t
+if_getqflushfn(if_t ifp)
+{
+	return (ifp->if_qflush);
+}
+
 void
 if_setsndtagallocfn(if_t ifp, if_snd_tag_alloc_t alloc_fn)
 {

sys/net/if_var.h

@@ -738,6 +738,7 @@ if_start_fn_t if_getstartfn(if_t ifp);
 void if_settransmitfn(if_t ifp, if_transmit_fn_t);
 if_transmit_fn_t if_gettransmitfn(if_t ifp);
 void if_setqflushfn(if_t ifp, if_qflush_fn_t);
+if_qflush_fn_t if_getqflushfn(if_t ifp);
 void if_setgetcounterfn(if_t ifp, if_get_counter_t);
 void if_setsndtagallocfn(if_t ifp, if_snd_tag_alloc_t);
 void if_setdebugnet_methods(if_t, struct debugnet_methods *);