net: Extend struct flowi6 with multipath hash

Allow for functions that fill out the IPv6 flow info to also pass a hash
computed over the skb contents. The hash value will drive the multipath
routing decisions.

This is intended for special treatment of ICMPv6 errors, where we would
like to make a routing decision based on the flow identifying the
offending IPv6 datagram that triggered the error, rather than the flow
of the ICMP error itself.

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