kselftest: rtnetlink: add pause and pause on fail flag

'Pause' prompts the user to press Enter to continue running tests
once one test has finished. Pause on fail on prompts the user to press
enter only when a test fails.

Modifications to kci_test_addrlft() and kci_test_ipsec_offload()
ensure that whenever end_test is called, [$ret -ne 0] indicates
failure. This allows end_test to really easily implement pause on fail
functionality.

Signed-off-by: Daniel Mendes <dmendes@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>