mm/damon/core: handle NULL ctx parameter in damon_call()

Patch series "mm/damon: allow NULL or unstarted damon_ctx parameter for
damon_call()".

Callers of damon_call() should validate the damon_ctx object parameter. 
If it is NULL or never damon_start()-ed object, damon_call() could
dereference the NULL pointer or indefinitely hang.  Ensuring all callers
doing the validation correctly has turned out to be difficult.  Handle the
corner cases inside the core layer and remove callers' validations.

Patches 1 and 2 respectively allow passing NULL and not yet
damon_start()-ed ctx parameter to damon_call().  Patches 3 and 4 remove
the callers side validations in DMON_RECLAIM and DASMON_LRU_SORT,
respectively.


This patch (of 4):

When NULL damon_ctx pointer parameter is passed, damon_call() could do
NULL dereference.  The caller is responsible to avoid that.  It is easy to
forget, and there are many damon_call() callers.  Meanwhile, damon_call()
is never meant to be performance critical.  It uses mutex and completion. 
Add the NULL pointer check inside damon_call() so that callers can pass
the parameter without NULL checks.

Link: https://lore.kernel.org/20260903010334.93622-1-sj@kernel.org
Link: https://lore.kernel.org/20260903010334.93622-2-sj@kernel.org
Signed-off-by: SJ Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/damon/core.c

@@ -2216,6 +2216,8 @@ int damon_kdamond_pid(struct damon_ctx *ctx)
  */
 int damon_call(struct damon_ctx *ctx, struct damon_call_control *control)
 {
+	if (!ctx)
+		return -EINVAL;
 	if (!control->repeat)
 		init_completion(&control->completion);
 	control->canceled = false;