mm/damon/dbgfs: fix 'struct pid' leaks in 'dbgfs_target_ids_write()'

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: mm/damon/dbgfs: fix 'struct pid' leaks in 'dbgfs_target_ids_write()' DAMON debugfs interface increases the reference counts of 'struct pid's for targets from the 'target_ids' file write callback ('dbgfs_target_ids_write()'), but decreases the counts only in DAMON monitoring termination callback ('dbgfs_before_terminate()'). Therefore, when 'target_ids' file is repeatedly written without DAMON monitoring start/termination, the reference count is not decreased and therefore memory for the 'struct pid' cannot be freed. This commit fixes this issue by decreasing the reference counts when 'target_ids' is written.

References

• https://git.kernel.org/stable/c/ffe4a1ba1a82c416a6b3a09d46594f6a885ae141
• https://git.kernel.org/stable/c/ebb3f994dd92f8fb4d70c7541091216c1e10cb71