selftests: forwarding: Test multipath hashing on inner IP pkts for GRE tunnel

Add selftest scripts for multipath hashing on inner IP pkts when there
is a single GRE tunnel but there are multiple underlay routes to reach
the other end of the tunnel.

Four cases are covered in these scripts:
    - IPv4 inner, IPv4 outer
    - IPv6 inner, IPv4 outer
    - IPv4 inner, IPv6 outer
    - IPv6 inner, IPv6 outer

Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Stephen Suryaputra <ssuryaextr@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>