swim: optimize struct swim_task layout

Before the patch it was split in two parts by 1.5KB packet, and
in the constructor it was nullifying the whole volume. Obviously,
these were mistakes. The first problem breaks cache locality,
the second one flushes the cache.

src/lib/swim/swim_io.c

@@ -166,7 +166,8 @@ void
 swim_task_create(struct swim_task *task, swim_task_f complete,
 		 swim_task_f cancel, const char *desc)
 {
-	memset(task, 0, sizeof(*task));
+	/* Do not nullify the whole structure! It is too big. */
+	memset(task, 0, offsetof(struct swim_task, packet));
 	task->complete = complete;
 	task->cancel = cancel;
 	task->desc = desc;

src/lib/swim/swim_io.h

@@ -233,8 +233,6 @@ struct swim_task {
 	 * and it cancels all its tasks.
 	 */
 	swim_task_f cancel;
-	/** Packet to send. */
-	struct swim_packet packet;
 	/** Destination address. */
 	struct sockaddr_in dst;
 	/**
@@ -264,6 +262,8 @@ struct swim_task {
 	};
 	/** Link in the task pool. */
 	struct stailq_entry in_pool;
+	/** Packet to send. */
+	struct swim_packet packet;
 };
 
 /** Check if @a task is already scheduled. */