selftests/pidfd: Fix ksft print formats

Compiling pidfd selftest after adding a __printf() attribute to
ksft_print_msg() and ksft_test_result_pass() exposes -Wformat warnings
in error_report(), test_pidfd_poll_exec_thread(),
child_poll_exec_test(), test_pidfd_poll_leader_exit_thread(),
child_poll_leader_exit_test().

The ksft_test_result_pass() in error_report() expects a string but
doesn't provide any argument after the format string. All the other
calls to ksft_print_msg() in the functions mentioned above have format
strings that don't match with other passed arguments.

Fix format specifiers so they match the passed variables.

Add a missing variable to ksft_test_result_pass() inside
error_report() so it matches other cases in the switch statement.

Fixes: 2def297e

 ("pidfd: add tests for NSpid info in fdinfo")

Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>