mm/damon/stat: deallocate damon_call() failure leaking damon_ctx

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

mm/damon/stat: deallocate damon_call() failure leaking damon_ctx

damon_stat_start() always allocates the module's damon_ctx object

(damon_stat_context). Meanwhile, if damon_call() in the function fails,

the damon_ctx object is not deallocated. Hence, if the damon_call() is

failed, and the user writes Y to “enabled” again, the previously

allocated damon_ctx object is leaked.

This cannot simply be fixed by deallocating the damon_ctx object when

damon_call() fails. That's because damon_call() failure doesn't guarantee

the kdamond main function, which accesses the damon_ctx object, is

completely finished. In other words, if damon_stat_start() deallocates

the damon_ctx object after damon_call() failure, the not-yet-terminated

kdamond could access the freed memory (use-after-free).

Fix the leak while avoiding the use-after-free by keeping returning

damon_stat_start() without deallocating the damon_ctx object after

damon_call() failure, but deallocating it when the function is invoked

again and the kdamond is completely terminated. If the kdamond is not yet

terminated, simply return -EAGAIN, as the kdamond will soon be terminated.

The issue was discovered [1] by sashiko.