ip6_input: Double check for mapped v4 addresses

[Martin Pieuchot]

This look like a bad merge from 2000s.  This check is present twice in
ip6_input(), so let's remove the late one, ok?

Index: netinet6/ip6_input.c
===================================================================
RCS file: /cvs/src/sys/netinet6/ip6_input.c,v
retrieving revision 1.158
diff -u -p -r1.158 ip6_input.c
--- netinet6/ip6_input.c        19 Apr 2016 08:23:13 -0000      1.158
+++ netinet6/ip6_input.c        6 May 2016 13:01:34 -0000
@@ -600,21 +600,6 @@ ip6_input(struct mbuf *m)
        ip6 = mtod(m, struct ip6_hdr *);
 
        /*
-        * Malicious party may be able to use IPv4 mapped addr to confuse
-        * tcp/udp stack and bypass security checks (act as if it was from
-        * 127.0.0.1 by using IPv6 src ::ffff:127.0.0.1).  Be cautious.
-        *
-        * For SIIT end node behavior, you may want to disable the check.
-        * However, you will  become vulnerable to attacks using IPv4 mapped
-        * source.
-        */
-       if (IN6_IS_ADDR_V4MAPPED(&ip6->ip6_src) ||
-           IN6_IS_ADDR_V4MAPPED(&ip6->ip6_dst)) {
-               ip6stat.ip6s_badscope++;
-               goto bad;
-       }
-
-       /*
         * Tell launch routine the next header
         */
        ip6stat.ip6s_delivered++;