selftests/mm: add folio_split() and filemap_get_entry() race test

The added folio_split_race_test is a modified C port of the race condition
test from [1].  The test creates shmem huge pages, where the main thread
punches holes in the shmem to cause folio_split() in the kernel and a set
of 16 threads reads the shmem to cause filemap_get_entry() in the kernel. 
filemap_get_entry() reads the folio and xarray split by folio_split()
locklessly.  The original test[2] is written in rust and uses memfd (shmem
backed).  This C port uses shmem directly and use a single process.

Note: the initial rust to C conversion is done by Cursor.

Link: https://lore.kernel.org/all/CAKNNEtw5_kZomhkugedKMPOG-sxs5Q5OLumWJdiWXv+C9Yct0w@mail.gmail.com/ [1]
Link: https://github.com/dfinity/thp-madv-remove-test [2]
Link: https://lkml.kernel.org/r/20260323163717.184107-1-ziy@nvidia.com
Co-developed-by: Bas van Dijk <bas@dfinity.org>
Signed-off-by: Bas van Dijk <bas@dfinity.org>
Co-developed-by: Adam Bratschi-Kaye <adam.bratschikaye@dfinity.org>
Signed-off-by: Adam Bratschi-Kaye <adam.bratschikaye@dfinity.org>
Signed-off-by: Zi Yan <ziy@nvidia.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Nico Pache <npache@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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