selftests/mm: split_huge_page_test: cleanups for split_pte_mapped_thp test

There is room for improvement, so let's clean up a bit:

(1) Define "4" as a constant.

(2) SKIP if we fail to allocate all THPs (e.g., fragmented) and add
    recovery code for all other failure cases: no need to exit the test.

(3) Rename "len" to thp_area_size, and "one_page" to "thp_area".

(4) Allocate a new area "page_area" into which we will mremap the
    pages; add "page_area_size". Now we can easily merge the two
    mremap instances into a single one.

(5) Iterate THPs instead of bytes when checking for missed THPs after
    mremap.

(6) Rename "pte_mapped2" to "tmp", used to verify mremap(MAP_FIXED)
    result.

(7) Split the corruption test from the failed-split test, so we can just
    iterate bytes vs. thps naturally.

(8) Extend comments and clarify why we are using mremap in the first
    place.

Link: https://lkml.kernel.org/r/20250903070253.34556-3-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Mariano Pache <npache@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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