netdevsim: fix rtnetlink.sh selftest

I cleared IFF_NOARP flag from netdevsim dev->flags in order to support
skb forwarding. This breaks the rtnetlink.sh selftest
kci_test_ipsec_offload() test because ipsec does not connect to peers it
cannot transmit to.

Fix the issue by adding a neigh entry manually. ipsec_offload test now
successfully pass.

Signed-off-by: David Wei <dw@davidwei.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>