mm/damon/sysfs-schemes: protect memcg_path kfree() with damon_sysfs_lock

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

mm/damon/sysfs-schemes: protect memcg_path kfree() with damon_sysfs_lock

Patch series "mm/damon/sysfs-schemes: fix use-after-free for [memcg_]path".

Reads of 'memcg_path' and 'path' files in DAMON sysfs interface could race

with their writes, results in use-after-free. Fix those.

This patch (of 2):

damon_sysfs_scheme_filter->mmecg_path can be read and written by users,

via DAMON sysfs memcg_path file. It can also be indirectly read, for the

parameters {on,off}line committing to DAMON. The reads for parameters

committing are protected by damon_sysfs_lock to avoid the sysfs files

being destroyed while any of the parameters are being read. But the

user-driven direct reads and writes are not protected by any lock, while

the write is deallocating the memcg_path-pointing buffer. As a result,

the readers could read the already freed buffer (user-after-free). Note

that the user-reads don't race when the same open file is used by the

writer, due to kernfs's open file locking. Nonetheless, doing the reads

and writes with separate open files would be common. Fix it by protecting

both the user-direct reads and writes with damon_sysfs_lock.