mm/damon/sysfs: use DAMON core API damon_is_running()

DAMON core implements a static function to see if a given DAMON context is
running.  DAMON sysfs interface is implementing the same one on its own. 
Make the core function non-static and reuse it from the DAMON sysfs
interface.

Link: https://lkml.kernel.org/r/20250705175000.56259-5-sj@kernel.org


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