mm/damon/core: get memcg reference before access

mm/damon/core: get memcg reference before access

The commit b74a120bcf507 ("mm/damon/core: implement
DAMOS_QUOTA_NODE_MEMCG_USED_BP") added accesses to memcg structure without
getting reference to it.  This is unsafe.  Let's get the reference before
accessing the memcg.

Link: https://lkml.kernel.org/r/20251225002904.139543-1-shakeel.butt@linux.dev
Fixes: b74a120bcf507 ("mm/damon/core: implement DAMOS_QUOTA_NODE_MEMCG_USED_BP")
Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
Reviewed-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>