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.