selftests/mm: add a new test for madv and hugetlb

Create a selftest that exercises the race between page faults and
madvise(MADV_DONTNEED) in the same huge page. Do it by running two
threads that touches the huge page and madvise(MADV_DONTNEED) at the same
time.

In case of a SIGBUS coming at pagefault, the test should fail, since we
hit the bug.

The test doesn't have a signal handler, and if it fails, it fails like
the following

  ----------------------------------
  running ./hugetlb_fault_after_madv
  ----------------------------------
  ./run_vmtests.sh: line 186: 595563 Bus error    (core dumped) "$@"
  [FAIL]

This selftest goes together with the fix of the bug[1] itself.

[1] https://lore.kernel.org/all/[email protected]/#r

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Breno Leitao <[email protected]>
Reviewed-by: Rik van Riel <[email protected]>
Tested-by: Rik van Riel <[email protected]>
Cc: Mike Kravetz <[email protected]>
Cc: Muchun Song <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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