selftests/mm: set allocated memory to non-zero content in cow test

After commit b1f20206 ("mm: remap unused subpages to shared zeropage
when splitting isolated thp"), cow test cases involving swapping out THPs
via madvise(MADV_PAGEOUT) started to be skipped due to the subsequent
check via pagemap determining that the memory was not actually swapped
out.  Logs similar to this were emitted:

   ...

   # [RUN] Basic COW after fork() ... with swapped-out, PTE-mapped THP (16 kB)
   ok 2 # SKIP MADV_PAGEOUT did not work, is swap enabled?
   # [RUN] Basic COW after fork() ... with single PTE of swapped-out THP (16 kB)
   ok 3 # SKIP MADV_PAGEOUT did not work, is swap enabled?
   # [RUN] Basic COW after fork() ... with swapped-out, PTE-mapped THP (32 kB)
   ok 4 # SKIP MADV_PAGEOUT did not work, is swap enabled?

   ...

The commit in question introduces the behaviour of scanning THPs and if
their content is predominantly zero, it splits them and replaces the pages
which are wholly zero with the zero page.  These cow test cases were
getting caught up in this.

So let's avoid that by filling the contents of all allocated memory with
a non-zero value. With this in place, the tests are passing again.

Link: https://lkml.kernel.org/r/20250107142555.1870101-1-ryan.roberts@arm.com


Fixes: b1f20206 ("mm: remap unused subpages to shared zeropage when splitting isolated thp")
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Usama Arif <usamaarif642@gmail.com>
Cc: Yu Zhao <yuzhao@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>