selftests: rtnetlink: use a local IP address for IPsec tests

Find an IP address on this machine to use as a source IP, and
make up a destination IP address based on the source IP.  No
actual messages will be sent, just a couple of IPsec rules are
created and deleted.

Fixes: 5e596ee1

 ("selftests: add xfrm state-policy-monitor to rtnetlink.sh")
Reported-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Shannon Nelson <shannon.nelson@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>