selftests/mm: hugepage_settings: add APIs for HugeTLB setup and teardown

A lot of tests require free HugeTLB pages.  Some need just a few default
huge pages, some need a certain amount of memory available as HugeTLB, and
some just skip lots of tests if huge pages of all supported sizes are not
available.

This all resulted in a huge mess in run_vmtests.sh that sets up some huge
pages, adjusts them later and restores some of the settings if the stars
align.

Add APIs that allow saving the state of HugeTLB and setting up the desired
amount of HugeTLB pages.

Saving the state also registers atexit() callback and signal handler that
will ensure restoration of HugeTLB state.

Since many tests use both HugeTLB and THP, the atexit() callbacks and
signal handler are restoring both.

For kselftest_harness tests that run fixture setups and test in child
processes add a constructor that will save and restore settings in the
main process.

Link: https://lore.kernel.org/20260511162840.375890-29-rppt@kernel.org
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Tested-by: Luiz Capitulino <luizcap@redhat.com>
Tested-by: Sarthak Sharma <sarthak.sharma@arm.com>
Reviewed-by: Luiz Capitulino <luizcap@redhat.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: Donet Tom <donettom@linux.ibm.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Leon Romanovsky <leon@kernel.org>
Cc: Liam Howlett <liam@infradead.org>
Cc: Li Wang <li.wang@linux.dev>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Nico Pache <npache@redhat.com>
Cc: Peter Xu <peterx@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>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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