net: ipv4,ipv6: Pass multipath hash computation through a helper

net: ipv4,ipv6: Pass multipath hash computation through a helper

The following patches will add a sysctl to control multipath hash
seed. In order to centralize the hash computation, add a helper,
fib_multipath_hash_from_keys(), and have all IPv4 and IPv6 route.c
invocations of flow_hash_from_keys() go through this helper instead.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20240607151357.421181-2-petrm@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>