mm/damon/core: split out scheme stat update logic into a new function

The function for applying a given DAMON scheme action to a given DAMON
region, 'damos_apply_scheme()' is not quite short.  Make it better to read
by splitting out the stat update logic into a new function.

Link: https://lkml.kernel.org/r/20221026225943.100429-4-sj@kernel.org


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