selftests: net: fix IOAM test skip return code

The ioam6.sh test script exits with an error code (1) when tests are
skipped due to lack of support from userspace/kernel or not enough
permissions. It should return the kselftests SKIP code instead.

Reviewed-by: Justin Iurman <justin.iurman@uliege.be>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Link: https://lore.kernel.org/r/20220801124615.256416-1-kleber.souza@canonical.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>