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 <[e-mail]>
Acked-by: Christian Brauner <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/pidfd/pidfd_test.c

···
 		ksft_exit_fail_msg("%s test: Failed to recycle pid %d\n",
 				   test_name, PID_RECYCLE);
 	case PIDFD_SKIP:
-		ksft_print_msg("%s test: Skipping test\n", test_name);
+		ksft_test_result_skip("%s test: Skipping test\n", test_name);
 		ret = 0;
 		break;
 	case PIDFD_XFAIL: