selftests: pidfd: do not use ksft_exit_skip after ksft_set_plan

Calling ksft_exit_skip after ksft_set_plan results in executing fewer tests
than planned.  Use ksft_test_result_skip instead.

The plan passed to ksft_set_plan was wrong, too, so fix it while at it.

Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>