mm/damon/sysfs-schemes: implement file for quota goal nid parameter

mm/damon/sysfs-schemes: implement file for quota goal nid parameter

DAMOS_QUOTA_NODE_MEM_{USED,FREE}_BP DAMOS quota goal metrics require the
node id parameter.  However, there is no DAMON user ABI for setting it.
Implement a DAMON sysfs file for that with name 'nid', under the quota
goal directory.

Link: https://lkml.kernel.org/r/20250420194030.75838-3-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Yunjeong Mun <yunjeong.mun@sk.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>