ipv6: Use multipath hash from flow info if available

Allow our callers to influence the choice of ECMP link by honoring the
hash passed together with the flow info. This allows for special
treatment of ICMP errors which we would like to route over the same path
as the IPv6 datagram that triggered the error.

Also go through rt6_multipath_hash(), in the usual case when we aren't
dealing with an ICMP error, so that there is one central place where
multipath hash is computed.

Signed-off-by: Jakub Sitnicki <jkbs@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>