selftests/mm: add PAGEMAP_SCAN test for THP PMD holes

Add coverage for the PMD-hole case fixed by "fs/proc/task_mmu: fix
PAGEMAP_SCAN written state for PMD holes": a MAP_PRIVATE|MAP_ANON THP that
is uffd-wp'd and then dropped with MADV_DONTNEED leaves a pmd_none hole
with no page table, which PAGEMAP_SCAN must still report as written.

Factor the populate/drop/scan-both-paths sequence out of
unpopulated_scan_test() into a helper, and add unpopulated_thp_scan_test()
that reuses it with a THP.

Include <linux/mman.h> for MADV_COLLAPSE; <sys/mman.h> lacks it on older
glibc (e.g. 2.34). Same approach as commit fd5295afae91 ("selftests/mm:
hmm-tests: include linux/mman.h to access MADV_COLLAPSE").

Link: https://lore.kernel.org/aljWYfPRCVc6IB2b@thinkstation
Link: https://lore.kernel.org/20260715144234.442721-3-kirill@shutemov.name
Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
Cc: Muhammad Usama Anjum <usama.anjum@arm.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Zenghui Yu <zenghui.yu@linux.dev>
Cc: David Hildenbrand <david@kernel.org>
Cc: Jann Horn <jannh@google.com>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Pedro Falcato <pfalcato@suse.de>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Assisted-by: Claude:claude-fable-5
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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