sched_ext: Use sizeof_field for key_len in dsq_hash_params

Update the `dsq_hash_params` initialization to use `sizeof_field`
for the `key_len` field instead of a hardcoded value.

This improves code readability and ensures the key length dynamically
matches the size of the `id` field in the `scx_dispatch_q` structure.

Signed-off-by: Liang Jie <liangjie@lixiang.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

Author: Liang Jie

kernel/sched/ext.c

@@ -960,7 +960,7 @@ static DEFINE_PER_CPU(struct task_struct *, direct_dispatch_task);
 static struct scx_dispatch_q **global_dsqs;
 
 static const struct rhashtable_params dsq_hash_params = {
-	.key_len		= 8,
+	.key_len		= sizeof_field(struct scx_dispatch_q, id),
 	.key_offset		= offsetof(struct scx_dispatch_q, id),
 	.head_offset		= offsetof(struct scx_dispatch_q, hash_node),
 };