mm/damon/sysfs: split out kdamond-independent schemes stats update logic into a new function

'damon_sysfs_schemes_update_stats()' is coupled with both
damon_sysfs_kdamond and damon_sysfs_schemes.  It's a wide range of types
dependency.  It makes splitting the logics a little bit distracting. 
Split the function so that each function is coupled with smaller range of
types.

Link: https://lkml.kernel.org/r/20221026225943.100429-10-sj@kernel.org


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