mm: mm_event: add compaction stat

mm: mm_event: add compaction stat

This patch adds compaction mm_event stat so that we could keep track
latency of compaction as well as count of the event.

Under heavy memory fragmentation, high-order page allocation(e.g.
fork, ION memory allocation) triggers compaction, which is
another major part of latency. Let's track it down, too.

Mot-CRs-fixed: (CR)

Bug: 80168800
Change-Id: Ia3da9324f123ba2542863eafaf72024b5351785b
Signed-off-by: Minchan Kim <minchan@google.com>
Reviewed-on: https://gerrit.mot.com/1453722
SLTApproved: Slta Waiver
SME-Granted: SME Approvals Granted
Tested-by: Jira Key
Reviewed-by: Xiangpo Zhao <zhaoxp3@motorola.com>
Submit-Approved: Jira Key