avoid byteswapping at runtime in tun(4)

[David Gwynne]

this has reads from tun load the AF out of the data rather than point to
it, then has the switch statement compare to the swapped AF values.

ok?

Index: if_tun.c
===================================================================
RCS file: /cvs/src/sys/net/if_tun.c,v
retrieving revision 1.184
diff -u -p -r1.184 if_tun.c
--- if_tun.c    3 Feb 2019 23:04:49 -0000       1.184
+++ if_tun.c    4 Feb 2019 00:18:54 -0000
@@ -833,7 +833,7 @@ int
 tun_dev_write(struct tun_softc *tp, struct uio *uio, int ioflag)
 {
        struct ifnet            *ifp;
-       u_int32_t               *th;
+       u_int32_t               th;
        struct mbuf             *top, **mp, *m;
        int                     error = 0, tlen;
        size_t                  mlen;
@@ -842,7 +842,7 @@ tun_dev_write(struct tun_softc *tp, stru
        TUNDEBUG(("%s: tunwrite\n", ifp->if_xname));
 
        if (uio->uio_resid == 0 || uio->uio_resid > ifp->if_mtu +
-           (tp->tun_flags & TUN_LAYER2 ? ETHER_HDR_LEN : sizeof(*th))) {
+           (tp->tun_flags & TUN_LAYER2 ? ETHER_HDR_LEN : sizeof(th))) {
                TUNDEBUG(("%s: len=%d!\n", ifp->if_xname, uio->uio_resid));
                return (EMSGSIZE);
        }
@@ -917,11 +917,11 @@ tun_dev_write(struct tun_softc *tp, stru
        }
 #endif
 
-       th = mtod(top, u_int32_t *);
+       th = *mtod(top, u_int32_t *);
        /* strip the tunnel header */
-       top->m_data += sizeof(*th);
-       top->m_len  -= sizeof(*th);
-       top->m_pkthdr.len -= sizeof(*th);
+       top->m_data += sizeof(th);
+       top->m_len  -= sizeof(th);
+       top->m_pkthdr.len -= sizeof(th);
        top->m_pkthdr.ph_rtableid = ifp->if_rdomain;
        top->m_pkthdr.ph_ifidx = ifp->if_index;
 
@@ -930,17 +930,17 @@ tun_dev_write(struct tun_softc *tp, stru
 
        NET_LOCK();
 
-       switch (ntohl(*th)) {
-       case AF_INET:
+       switch (th) {
+       case htonl(AF_INET):
                ipv4_input(ifp, top);
                break;
 #ifdef INET6
-       case AF_INET6:
+       case htonl(AF_INET6):
                ipv6_input(ifp, top);
                break;
 #endif
 #ifdef MPLS
-       case AF_MPLS:
+       case htonl(AF_MPLS):
                mpls_input(ifp, top);
                break;
 #endif