selftest/mm: make hugetlb_reparenting_test tolerant to async reparenting

selftest/mm: make hugetlb_reparenting_test tolerant to async reparenting

In cgroup v2, memory and hugetlb usage reparenting is asynchronous.  This
can cause test flakiness when immediately asserting usage after deleting a
child cgroup.  To address this, add a helper function
`assert_with_retry()` that checks usage values with a timeout-based retry.
This improves test stability without relying on fixed sleep delays.

Also bump up the tolerance size to 7MB.

To avoid False Positives:
  ...
  # Assert memory charged correctly for child only use.
  # actual a = 11 MB
  # expected a = 0 MB
  # fail
  # cleanup
  # [FAIL]
  not ok 11 hugetlb_reparenting_test.sh -cgroup-v2 # exit=1
  # 0
  # SUMMARY: PASS=10 SKIP=0 FAIL=1

Link: https://lkml.kernel.org/r/20250407084201.74492-1-liwang@redhat.com
Signed-off-by: Li Wang <liwang@redhat.com>
Tested-by: Donet Tom <donettom@linux.ibm.com>
Cc: Waiman Long <longman@redhat.com>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Kirill A. Shuemov <kirill.shutemov@linux.intel.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>