selftests: forwarding: Move multipath_eval() to lib.sh

This function will be useful for the GRE multipath test that is coming
later.

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