ipv6: Refactor update of IPv6 flowi destination address for srcrt (RH) option

There are more than a dozen occurrences of following code in the
IPv6 stack:

    if (opt && opt->srcrt) {
            struct rt0_hdr *rt0 = (struct rt0_hdr *) opt->srcrt;
            ipv6_addr_copy(&final, &fl.fl6_dst);
            ipv6_addr_copy(&fl.fl6_dst, rt0->addr);
            final_p = &final;
    }

Replace those with a helper. Note that the helper overrides final_p
in all cases. This is ok as final_p was previously initialized to
NULL when declared.

Signed-off-by: Arnaud Ebalard <arno@natisbad.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/ipv6.h

@@ -551,6 +551,10 @@ extern int 			ipv6_ext_hdr(u8 nexthdr);
 
 extern int ipv6_find_tlv(struct sk_buff *skb, int offset, int type);
 
+extern struct in6_addr *fl6_update_dst(struct flowi *fl,
+				       const struct ipv6_txoptions *opt,
+				       struct in6_addr *orig);
+
 /*
  *	socket options (ipv6_sockglue.c)
  */