mm/compaction: make proactive compaction high watermark configurable via sysctl

mm/compaction: make proactive compaction high watermark configurable via sysctl

Currently, the difference between the high and low watermarks for
proactive compaction is hardcoded to 10.  This hardcoded difference is too
large for free page reporting to work well.

Add a new sysctl, `compaction_proactiveness_leeway`, to control the
difference between the high and low watermarks.

Link: https://lkml.kernel.org/r/20250127215020.4023545-3-mclapinski@google.com
Signed-off-by: Michal Clapinski <mclapinski@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>