selftest/net/forwarding: declare NETIFS p9 p10

The recent GRE selftests defined NUM_NETIFS=10. If the users copy
forwarding.config.sample to forwarding.config directly, they will get
error "Command line is not complete" when run the GRE tests, because
create_netif_veth() failed with no interface name defined.

Fix it by extending the NETIFS with p9 and p10.

Fixes: 2800f248 ("selftests: forwarding: Test multipath hashing on inner IP pkts for GRE tunnel")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>