selftests/pidfd: Fix duplicate-symbol warnings for SCHED_ CPP symbols

selftests/pidfd: Fix duplicate-symbol warnings for SCHED_ CPP symbols

The pidfd selftests run in userspace and include both userspace and kernel
header files.  On some distros (for example, CentOS), this results in
duplicate-symbol warnings in allmodconfig builds, while on other distros
(for example, Ubuntu) it does not.

Therefore, use #undef to get rid of the userspace definitions in favor
of the kernel definitions.

Other ways of handling this include splitting up the selftest code so
that the userspace definitions go into one translation unit and the
kernel definitions into another (which might or might not be feasible)
or to adjust compiler command-line options to suppress the warnings
(which might or might not be desirable).

[ paulmck: Apply Shuah Khan feedback. ]

Link: https://lore.kernel.org/r/cc7e4fe7-299f-4bf3-af46-df6551d61997@paulmck-laptop
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Shuah Khan <skhan@linuxfoundation.org>
Cc: Christian Brauner <brauner@kernel.org>
Cc: <linux-kselftest@vger.kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>