selftests: rtnetlink: skip ipsec offload tests if netdevsim isn't present

running the script on systems without netdevsim now prints:

SKIP: ipsec_offload can't load netdevsim

instead of error message & failed status.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>