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

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

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