mm/damon: fix divide by zero in damon_get_intervals_score()

The current implementation allows having zero size regions with no special
reasons, but damon_get_intervals_score() gets crashed by divide by zero
when the region size is zero.

  [   29.403950] Oops: divide error: 0000 [#1] SMP NOPTI

This patch fixes the bug, but does not disallow zero size regions to keep
the backward compatibility since disallowing zero size regions might be a
breaking change for some users.

In addition, the same crash can happen when intervals_goal.access_bp is
zero so this should be fixed in stable trees as well.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: f04b0fe ("mm/damon/core: implement intervals auto-tuning")
Signed-off-by: Honggyu Kim <[email protected]>
Reviewed-by: SeongJae Park <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: honggyukim

mm/damon/core.c

@@ -1449,6 +1449,7 @@ static unsigned long damon_get_intervals_score(struct damon_ctx *c)
 		}
 	}
 	target_access_events = max_access_events * goal_bp / 10000;
+	target_access_events = target_access_events ? : 1;
 	return access_events * 10000 / target_access_events;
 }