mlxsw: spectrum_outer: Factor out helper for common outer fields

Outer IPv4 and IPv6 addresses are used by multiple multipath hash
policies. Factor out helpers that set these fields to increase code
sharing between different policies.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>