use rtalloc_mpath in pf_route{,6}

[David Gwynne]

if you have multiple links to the same destination, this will let you
use them via route-to/reply-to/dup-to.

ok?

Index: pf.c
===================================================================
RCS file: /cvs/src/sys/net/pf.c,v
retrieving revision 1.1110
diff -u -p -r1.1110 pf.c
--- pf.c        12 Feb 2021 16:16:10 -0000      1.1110
+++ pf.c        15 Feb 2021 09:59:50 -0000
@@ -6020,7 +6020,7 @@ pf_route(struct pf_pdesc *pd, struct pf_
        dst->sin_addr = s->rt_addr.v4;
        rtableid = m0->m_pkthdr.ph_rtableid;
 
-       rt = rtalloc(sintosa(dst), RT_RESOLVE, rtableid);
+       rt = rtalloc_mpath(sintosa(dst), &ip->ip_src.s_addr, rtableid);
        if (!rtisvalid(rt)) {
                if (s->rt != PF_DUPTO) {
                        pf_send_icmp(m0, ICMP_UNREACH, ICMP_UNREACH_HOST,
@@ -6162,7 +6162,8 @@ pf_route6(struct pf_pdesc *pd, struct pf
        dst->sin6_addr = s->rt_addr.v6;
        rtableid = m0->m_pkthdr.ph_rtableid;
 
-       rt = rtalloc(sin6tosa(dst), RT_RESOLVE, rtableid);
+       rt = rtalloc_mpath(sin6tosa(dst), &ip6->ip6_src.s6_addr32[0],
+           rtableid);
        if (!rtisvalid(rt)) {
                if (s->rt != PF_DUPTO) {
                        pf_send_icmp(m0, ICMP6_DST_UNREACH,