mm/damon/dbgfs: protect targets destructions with kdamond_lock

In the Linux kernel, the following vulnerability has been resolved:

mm/damon/dbgfs: protect targets destructions with kdamond_lock

DAMON debugfs interface iterates current monitoring targets in 'dbgfs_target_ids_read()' while holding the corresponding 'kdamond_lock'. However, it also destructs the monitoring targets in 'dbgfs_before_terminate()' without holding the lock. This can result in a use_after_free bug. This commit avoids the race by protecting the destruction with the corresponding 'kdamond_lock'.