selftests/mm: don't compare return values to in cow

selftests/mm: don't compare return values to in cow

Tweak the coding style for checking for non-zero return values.
While we're at it also remove a now redundant oring of the madvise()
return code.

Link: https://lkml.kernel.org/r/20250610-selftest-mm-cow-tweaks-v1-3-43cd7457500f@kernel.org
Signed-off-by: Mark Brown <broonie@kernel.org>
Suggested-by: David Hildenbrand <david@redhat.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>