mm/damon/core: implement damon_initialized() function

Patch series "mm/damon: define and use DAMON initialization check
function".

DAMON is initialized in subsystem initialization time, by damon_init().
If DAMON API functions are called before the initialization, the
system could crash. Actually such issues happened and were fixed [1]
in the past. For the fix, DAMON API callers have updated to check if
DAMON is initialized or not, using their own hacks. The hacks are
unnecessarily duplicated on every DAMON API callers and therefore it
would be difficult to reliably maintain in the long term.

Make it reliable and easy to maintain. For this, implement a new DAMON
core layer API function that returns if DAMON is successfully
initialized. If it returns true, it means DAMON API functions are safe
to be used. After the introduction of the new API, update DAMON API
callers to use the new function instead of their own hacks.


This patch (of 7):

If DAMON is tried to be used when it is not yet successfully initialized,
the caller could be crashed. DAMON core layer is not providing a reliable
way to see if it is successfully initialized and therefore ready to be
used, though. As a result, DAMON API callers are implementing their own
hacks to see it. The hacks simply assume DAMON should be ready on module
init time. It is not reliable as DAMON initialization can indeed fail if
KMEM_CACHE() fails, and difficult to maintain as those are duplicates.
Implement a core layer API function for better reliability and
maintainability to replace the hacks with followup commits.

Link: https://lkml.kernel.org/r/20250916033511.116366-2-sj@kernel.org
Link: https://lkml.kernel.org/r/20250916033511.116366-2-sj@kernel.org
Link: https://lore.kernel.org/20250909022238.2989-1-sj@kernel.org [1]
Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>