mm/damon/core: introduce damon_call()

mm/damon/core: introduce damon_call()

Introduce a new DAMON core API function, damon_call().  It aims to replace
some damon_callback usages that access damon_ctx of ongoing kdamond with
additional synchronizations.  It receives a function pointer, let the
parallel kdamond invokes the function, and returns after the invocation is
finished, or canceled due to some races.

kdamond invokes the function inside the main loop after sampling is done.
If it is deactivated by DAMOS watermarks or already out of the main loop,
mark the request as canceled so that damon_call() can wakeup and return.

Link: https://lkml.kernel.org/r/20250103174400.54890-4-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>