simplify vxlan_lookup return value handling

[Mike Belopuhov]

After my previous commit to the vxlan(4) driver it can
no longer return -1 making this code path obsolete.
OK to remove it?

diff --git sys/netinet/udp_usrreq.c sys/netinet/udp_usrreq.c
index 60a7bdf..fc5e866 100644
--- sys/netinet/udp_usrreq.c
+++ sys/netinet/udp_usrreq.c
@@ -388,17 +388,12 @@ udp_input(struct mbuf *m, ...)
 #if NVXLAN > 0
        if (vxlan_enable > 0 &&
 #if NPF > 0
            !(m->m_pkthdr.pf.flags & PF_TAG_DIVERTED) &&
 #endif
-           (error = vxlan_lookup(m, uh, iphlen, &srcsa.sa, &dstsa.sa)) != 0) {
-               if (error == -1) {
-                       udpstat.udps_hdrops++;
-                       m_freem(m);
-               }
+           (error = vxlan_lookup(m, uh, iphlen, &srcsa.sa, &dstsa.sa)) != 0)
                return;
-       }
 #endif
 
        if (m->m_flags & (M_BCAST|M_MCAST)) {
                struct inpcb *last;
                /*