mm/damon/core: add cleanup_target() ops callback

mm/damon/core: add cleanup_target() ops callback

Some DAMON operation sets may need additional cleanup per target.  For
example, [f]vaddr need to put pids of each target.  Each user and core
logic is doing that redundantly.  Add another DAMON ops callback that will
be used for doing such cleanups in operations set layer.

[sj@kernel.org: add kernel-doc comment for damon_operations->cleanup_target]
Link: https://lkml.kernel.org/r/20250715185239.89152-2-sj@kernel.org
[sj@kernel.org: remove damon_ctx->callback kernel-doc comment]
Link: https://lkml.kernel.org/r/20250715185239.89152-3-sj@kernel.org
Link: https://lkml.kernel.org/r/20250712195016.151108-10-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>