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

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

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

damon_sysfs_quot_goal->path can be read and written by users, via DAMON

sysfs '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 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.