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: 2800f2485417 ("selftests: forwarding: Test multipath hashing on inner IP pkts for GRE tunnel")
Signed-off-by: Hangbin Liu <[e-mail]>
Reviewed-by: Ido Schimmel <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

tools/testing/selftests/net/forwarding/forwarding.config.sample

···
 NETIFS[p6]=veth5
 NETIFS[p7]=veth6
 NETIFS[p8]=veth7
+NETIFS[p9]=veth8
+NETIFS[p10]=veth9
 
 # Port that does not have a cable connected.
 NETIF_NO_CABLE=eth8