selftests: pidfd: skip test on kcmp() ENOSYS

Skip test if kcmp() is not available, for example if kernel is compiled
without CONFIG_CHECKPOINT_RESTORE=y.

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>