ipv6: Compute multipath hash for ICMP errors from offending packet

When forwarding or sending out an ICMPv6 error, look at the embedded
packet that triggered the error and compute a flow hash over its
headers.

This let's us route the ICMP error together with the flow it belongs to
when multipath (ECMP) routing is in use, which in turn makes Path MTU
Discovery work in ECMP load-balanced or anycast setups (RFC 7690).

Granted, end-hosts behind the ECMP router (aka servers) need to reflect
the IPv6 Flow Label for PMTUD to work.

The code is organized to be in parallel with ipv4 stack:

  ip_multipath_l3_keys -> ip6_multipath_l3_keys
  fib_multipath_hash   -> rt6_multipath_hash

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