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

In the Linux kernel, the following vulnerability has been resolved: 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.