mm/compaction.c: clear total_{migrate,free}_scanned before scanning a new zone

mm/compaction.c: clear total_{migrate,free}_scanned before scanning a new zone

total_{migrate,free}_scanned will be added to COMPACTMIGRATE_SCANNED and
COMPACTFREE_SCANNED in compact_zone().  We should clear them before
scanning a new zone.  In the proc triggered compaction, we forgot clearing
them.

[[email protected]: introduce a helper compact_zone_counters_init()]
Link: http://lkml.kernel.org/r/[email protected]
[[email protected]: expand compact_zone_counters_init() into its single callsite, per mhocko]
[[email protected]: squash compact_zone() list_head init as well]
Link: http://lkml.kernel.org/r/[email protected]
[[email protected]: kcompactd_do_work(): avoid unnecessary initialization of cc.zone]
Link: http://lkml.kernel.org/r/[email protected]
Fixes: 7f354a548d1c ("mm, compaction: add vmstats for kcompactd work")
Signed-off-by: Yafang Shao <[email protected]>
Signed-off-by: Vlastimil Babka <[email protected]>
Reviewed-by: Vlastimil Babka <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Yafang Shao <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>