mm/damon/sysfs: move targets setup code to a separated function

This commit separates DAMON sysfs interface's monitoring context targets
setup code to a new function for better readability.

Link: https://lkml.kernel.org/r/20220429160606.127307-8-sj@kernel.org


Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>