selftests: pidfd: use ksft_test_result_skip() when skipping test

There's planned tests != run tests in pidfd_test when some test is
skipped:

  $ ./pidfd_test
  TAP version 13
  1..8
  [...]
  # pidfd_send_signal signal recycled pid test: Skipping test
  # Planned tests != run tests (8 != 7)
  # Totals: pass:7 fail:0 xfail:0 xpass:0 skip:0 error:0

Fix by using ksft_test_result_skip():

  $ ./pidfd_test
  TAP version 13
  1..8
  [...]
  ok 8 # SKIP pidfd_send_signal signal recycled pid test: Unsharing pid namespace not permitted
  # Totals: pass:7 fail:0 xfail:0 xpass:0 skip:1 error:0

Signed-off-by: Tommi Rantala <tommi.t.rantala@nokia.com>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>