kernel: use list.h for DPC queues

kernel-src/include/kernel/dpc.h

@@ -3,13 +3,13 @@
 
 #include <arch/context.h>
 #include <stdbool.h>
+#include <list.h>
 
 typedef void * dpcarg_t;
 typedef void (*dpcfn_t)(context_t *, dpcarg_t);
 
 typedef struct _dpc_t {
-	struct _dpc_t *next;
-	struct _dpc_t *prev;
+	list_node_t list_node;
 	bool enqueued;
 	dpcfn_t fn;
 	dpcarg_t arg;

kernel-src/include/x86-64/arch/cpu.h

@@ -46,7 +46,7 @@ typedef struct cpu_t {
 	timer_t *timer;
 
 	isr_t *dpcisr;
-	dpc_t *dpcqueue;
+	list_t dpcqueue;
 
 	thread_t *idlethread;
 	timerentry_t schedtimerentry;