selftests: rtnetlink: clear the return code at start of ipsec test

Following the custom from the other functions, clear the global
ret code before starting the test so as to not have previously
failed tests cause us to thing this test has failed.

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>