mm/damon/sysfs: check contexts->nr in repeat_call_fn

damon_sysfs_repeat_call_fn() calls damon_sysfs_upd_tuned_intervals(),
damon_sysfs_upd_schemes_stats(), and
damon_sysfs_upd_schemes_effective_quotas() without checking contexts->nr. 
If nr_contexts is set to 0 via sysfs while DAMON is running, these
functions dereference contexts_arr[0] and cause a NULL pointer
dereference.  Add the missing check.

For example, the issue can be reproduced using DAMON sysfs interface and
DAMON user-space tool (damo) [1] like below.

    $ sudo damo start --refresh_interval 1s
    $ echo 0 | sudo tee \
            /sys/kernel/mm/damon/admin/kdamonds/0/contexts/nr_contexts

Link: https://patch.msgid.link/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Link: https://github.com/damonitor/damo [1]
Fixes: d809a7c ("mm/damon/sysfs: implement refresh_ms file internal work")
Signed-off-by: Josh Law <[email protected]>
Reviewed-by: SeongJae Park <[email protected]>
Signed-off-by: SeongJae Park <[email protected]>
Cc: <[email protected]>	[6.17+]
Signed-off-by: Andrew Morton <[email protected]>

Author: techyguyperplexable

mm/damon/sysfs.c

@@ -1620,9 +1620,12 @@ static int damon_sysfs_repeat_call_fn(void *data)
 
 	if (!mutex_trylock(&damon_sysfs_lock))
 		return 0;
+	if (sysfs_kdamond->contexts->nr != 1)
+		goto out;
 	damon_sysfs_upd_tuned_intervals(sysfs_kdamond);
 	damon_sysfs_upd_schemes_stats(sysfs_kdamond);
 	damon_sysfs_upd_schemes_effective_quotas(sysfs_kdamond);
+out:
 	mutex_unlock(&damon_sysfs_lock);
 	return 0;
 }