M_PREPEND in vxlan encap on not strict-alignment archs

[David Gwynne]

if the arch can cope with prepending on an unaligned address in
vxlan, then let it do it.

this means less work if we can get away with it.

ok?

Index: if_vxlan.c
===================================================================
RCS file: /cvs/src/sys/net/if_vxlan.c,v
retrieving revision 1.49
diff -u -p -r1.49 if_vxlan.c
--- if_vxlan.c  7 Oct 2016 06:16:03 -0000       1.49
+++ if_vxlan.c  10 Oct 2016 03:42:36 -0000
@@ -766,20 +765,29 @@ vxlan_output(struct ifnet *ifp, struct m
 #endif
        int                      error, af;
        uint32_t                 tag;
-       struct mbuf             *m0;
 
        /* VXLAN header */
-       MGETHDR(m0, M_DONTWAIT, m->m_type);
-       if (m0 == NULL) {
-               ifp->if_oerrors++;
-               return (ENOBUFS);
+       if (!ALIGNED_POINTER(mtod(m, caddr_t), uint32_t)) {
+               struct mbuf *m0;
+
+               MGETHDR(m0, M_DONTWAIT, m->m_type);
+               if (m0 == NULL) {
+                       ifp->if_oerrors++;
+                       return (ENOBUFS);
+               }
+               M_MOVE_PKTHDR(m0, m);
+               m0->m_next = m;
+               m = m0;
+               MH_ALIGN(m, sizeof(*vu));
+               m->m_len = sizeof(*vu);
+               m->m_pkthdr.len += sizeof(*vu);
+       } else {
+               M_PREPEND(m, sizeof(*vu), M_DONTWAIT);
+               if (m == NULL) {
+                       ifp->if_oerrors++;
+                       return (ENOBUFS);
+               }
        }
-       M_MOVE_PKTHDR(m0, m);
-       m0->m_next = m;
-       m = m0;
-       MH_ALIGN(m, sizeof(*vu));
-       m->m_len = sizeof(*vu);
-       m->m_pkthdr.len += sizeof(*vu);
 
        src = (struct sockaddr *)&sc->sc_src;
        dst = (struct sockaddr *)&sc->sc_dst;