selftests: rtnetlink: use dummydev as a test device

We really shouldn't mess with local system settings, so let's
use the already created dummy device instead for ipsec testing.
Oh, and let's put the temp file into a proper directory.

Signed-off-by: Shannon Nelson <shannon.nelson@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>