mm/damon: use damon_sz_region() in appropriate place

In many places we can use damon_sz_region() to instead of "r->ar.end -
r->ar.start".

Link: https://lkml.kernel.org/r/20220927001946.85375-2-xhao@linux.alibaba.com


Signed-off-by: Xin Hao <xhao@linux.alibaba.com>
Suggested-by: SeongJae Park <sj@kernel.org>
Reviewed-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>