selftests: cachestat: test for cachestat availability

Patch series "selftests: cachestat: fix run on older kernels", v2.

I ran all kernel selftests on some test machine, and stumbled upon
cachestat failing (among others).  These patches fix the run on older
kernels and when the current directory is on a tmpfs instance.


This patch (of 2):

As cachestat is a new syscall, it won't be available on older kernels, for
instance those running on a development machine.  At the moment the test
reports all tests as "not ok" in this case.

Test for the cachestat syscall availability first, before doing further
tests, and bail out early with a TAP SKIP comment.

This also uses the opportunity to add the proper TAP headers, and add one
check for proper error handling (illegal file descriptor).

Link: https://lkml.kernel.org/r/20230821160534.3414911-1-andre.przywara@arm.com
Link: https://lkml.kernel.org/r/20230821160534.3414911-2-andre.przywara@arm.com


Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Nhat Pham <nphamcs@gmail.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>