selftests/mm: compaction_test: fix bogus test success and reduce probability of OOM-killer invocation

Reset nr_hugepages to zero before the start of the test.

If a non-zero number of hugepages is already set before the start of the
test, the following problems arise:

 - The probability of the test getting OOM-killed increases.  Proof:
   The test wants to run on 80% of available memory to prevent OOM-killing
   (see original code comments).  Let the value of mem_free at the start
   of the test, when nr_hugepages = 0, be x.  In the other case, when
   nr_hugepages > 0, let the memory consumed by hugepages be y.  In the
   former case, the test operates on 0.8 * x of memory.  In the latter,
   the test operates on 0.8 * (x - y) of memory, with y already filled,
   hence, memory consumed is y + 0.8 * (x - y) = 0.8 * x + 0.2 * y > 0.8 *
   x.  Q.E.D

 - The probability of a bogus test success increases.  Proof: Let the
   memory consumed by hugepages be greater than 25% of x, with x and y
   defined as above.  The definition of compaction_index is c_index = (x -
   y)/z where z is the memory consumed by hugepages after trying to
   increase them again.  In check_compaction(), we set the number of
   hugepages to zero, and then increase them back; the probability that
   they will be set back to consume at least y amount of memory again is
   very high (since there is not much delay between the two attempts of
   changing nr_hugepages).  Hence, z >= y > (x/4) (by the 25% assumption).
   Therefore, c_index = (x - y)/z <= (x - y)/y = x/y - 1 < 4 - 1 = 3
   hence, c_index can always be forced to be less than 3, thereby the test
   succeeding always.  Q.E.D

Link: https://lkml.kernel.org/r/20240521074358.675031-4-dev.jain@arm.com
Fixes: bd67d5c1 ("Test compaction of mlocked memory")
Signed-off-by: Dev Jain <dev.jain@arm.com>
Cc: <stable@vger.kernel.org>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Sri Jayaramappa <sjayaram@akamai.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>