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>

tools/testing/selftests/net/rtnetlink.sh

@@ -801,6 +801,8 @@ kci_test_ipsec_offload()
 		end_test "FAIL: ipsec_offload SA offload missing from list output"
 	fi
 
+	# we didn't create a peer, make sure we can Tx
+	ip neigh add $dstip dev $dev lladdr 00:11:22:33:44:55
 	# use ping to exercise the Tx path
 	ping -I $dev -c 3 -W 1 -i 0 $dstip >/dev/null