pair: send without kernel lock

[Klemens Nanni]

pair(4)'s output path can run without kernel lock just fine.

NB: Looking at CVS log, it seems this was not done during import because
IFXF_MPSSAFE became a thing afterwards.

Feedback? Objections? OK?

Index: if_pair.c
===================================================================
RCS file: /cvs/src/sys/net/if_pair.c,v
retrieving revision 1.16
diff -u -p -r1.16 if_pair.c
--- if_pair.c   21 Aug 2020 22:59:27 -0000      1.16
+++ if_pair.c   7 Jan 2021 00:20:20 -0000
@@ -37,7 +37,7 @@
 
 void   pairattach(int);
 int    pairioctl(struct ifnet *, u_long, caddr_t);
-void   pairstart(struct ifnet *);
+void   pairstart(struct ifqueue *);
 int    pair_clone_create(struct if_clone *, int);
 int    pair_clone_destroy(struct ifnet *);
 int    pair_media_change(struct ifnet *);
@@ -116,8 +116,8 @@ pair_clone_create(struct if_clone *ifc, 
 
        ifp->if_softc = sc;
        ifp->if_ioctl = pairioctl;
-       ifp->if_start = pairstart;
-       ifp->if_xflags = IFXF_CLONED;
+       ifp->if_qstart = pairstart;
+       ifp->if_xflags = IFXF_CLONED | IFXF_MPSAFE;
 
        ifp->if_hardmtu = ETHER_MAX_HARDMTU_LEN;
        ifp->if_capabilities = IFCAP_VLAN_MTU;
@@ -158,8 +158,9 @@ pair_clone_destroy(struct ifnet *ifp)
 }
 
 void
-pairstart(struct ifnet *ifp)
+pairstart(struct ifqueue *ifq)
 {
+       struct ifnet            *ifp = ifq->ifq_if;
        struct pair_softc       *sc = (struct pair_softc *)ifp->if_softc;
        struct mbuf_list         ml = MBUF_LIST_INITIALIZER();
        struct ifnet            *pairedifp;
@@ -167,11 +168,7 @@ pairstart(struct ifnet *ifp)
 
        pairedifp = if_get(sc->sc_pairedif);
 
-       for (;;) {
-               m = ifq_dequeue(&ifp->if_snd);
-               if (m == NULL)
-                       break;
-
+       while ((m = ifq_dequeue(ifq)) != NULL) {
 #if NBPFILTER > 0
                if (ifp->if_bpf)
                        bpf_mtap(ifp->if_bpf, m, BPF_DIRECTION_OUT);