selftests/mm: new test that steals pages

This test stresses the race between of madvise(DONTNEED), a page fault
and a parallel huge page mmap, which should fail due to lack of
available page available for mapping.

This test case must run on a system with one and only one huge page
available.

	# echo 1 > /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages

During setup, the test allocates the only available page, and starts
three threads:

  - thread 1:
      * madvise(MADV_DONTNEED) on the allocated huge page
  - thread 2:
      * Write to the allocated huge page
  - thread 3:
      * Tries to allocated (steal) an extra huge page (which is not
        available)

thread 3 should never succeed in the allocation, since the only huge
page was never unmapped, and should be reserved.

Touching the old page after thread3 allocation will raise a SIGBUS.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Breno Leitao <[email protected]>
Cc: Mike Rapoport (IBM) <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: Vegard Nossum <[email protected]>
Cc: Yang Shi <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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