mm/damon/sysfs: don't hold kdamond_lock in before_terminate()

mm/damon/sysfs: don't hold kdamond_lock in before_terminate()

damon_sysfs_before_terminate() is a DAMON callback that is executed from
the kdamond's context.  Hence it is safe to access DAMON context internal
data.  But the function is unnecessarily holding kdamond_lock of the
context.  It is just unnecessary.  Remove the locking code.

Link: https://lkml.kernel.org/r/20250705175000.56259-6-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>