mm/damon/core: fix wasteful CPU calls by skipping non-existent targets

Currently, DAMON does not proactively clean up invalid monitoring targets
during its runtime. When some monitored processes exit, DAMON continues
to make the following unnecessary function calls,

--damon_for_each_target--
--damon_for_each_region--
damon_do_apply_schemes
damos_apply_scheme
damon_va_apply_scheme
damos_madvise
damon_get_mm

it is only in the damon_get_mm() function that it may finally discover the
target no longer exists, which wastes CPU resources. A simple idea is to
check for the existence of monitoring targets within the
kdamond_need_stop() function and promptly clean up non-existent targets.

However, SJ pointed out that this approach is problematic because the
online commit logic incorrectly uses list indices to update the monitoring
state. This can lead to data loss if the target list is changed
concurrently. Meanwhile, SJ suggests checking for target existence at the
damon_for_each_target level, and if a target does not exist, simply skip
it and proceed to the next one.

Link: https://lkml.kernel.org/r/20251210052508.264433-1-lienze@kylinos.cn
Signed-off-by: Enze Li <lienze@kylinos.cn>
Suggested-by: SeongJae Park <sj@kernel.org>
Reviewed-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>