selftests: add selftests for cachestat

Test cachestat on a newly created file, /dev/ files, /proc/ files and a
directory.  Also test on a shmem file (which can also be tested with
huge pages since tmpfs supports huge pages).

[colin.i.king@gmail.com: fix spelling mistake "trucate" -> "truncate"]
  Link: https://lkml.kernel.org/r/20230505110855.2493457-1-colin.i.king@gmail.com
[mpe@ellerman.id.au: avoid excessive stack allocation]
  Link: https://lkml.kernel.org/r/877ctfa6yv.fsf@mail.lhotse
Link: https://lkml.kernel.org/r/20230503013608.2431726-4-nphamcs@gmail.com
Signed-off-by: Nhat Pham <nphamcs@gmail.com>
Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Brian Foster <bfoster@redhat.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: Colin Ian King <colin.i.king@gmail.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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