selftests/mm: compaction_test: support platform with huge mount of memory

When running mm selftest to verify mm patches, 'compaction_test' case
failed on an x86 server with 1TB memory.  And the root cause is that it
has too much free memory than what the test supports.

The test case tries to allocate 100000 huge pages, which is about 200 GB
for that x86 server, and when it succeeds, it expects it's large than 1/3
of 80% of the free memory in system.  This logic only works for platform
with 750 GB ( 200 / (1/3) / 80% ) or less free memory, and may raise false
alarm for others.

Fix it by changing the fixed page number to self-adjustable number
according to the real number of free memory.

Link: https://lkml.kernel.org/r/20250423103645.2758-1-feng.tang@linux.alibaba.com


Fixes: bd67d5c1 ("Test compaction of mlocked memory")
Signed-off-by: Feng Tang <feng.tang@linux.alibaba.com>
Acked-by: Dev Jain <dev.jain@arm.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Tested-by: Baolin Wang <baolin.wang@inux.alibaba.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Sri Jayaramappa <sjayaram@akamai.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>