mm/damon/sysfs: cleanup attrs subdirs on context dir setup failure

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

mm/damon/sysfs: cleanup attrs subdirs on context dir setup failure

When a context DAMON sysfs directory setup is failed after setup of attrs/ directory, subdirectories of attrs/ directory are not cleaned up. As a result, DAMON sysfs interface is nearly broken until the system reboots, and the memory for the unremoved directory is leaked.

Cleanup the directories under such failures.