vhost: reduce stack usage in log_used

Fix the warning: [-Werror=-Wframe-larger-than=]

drivers/vhost/vhost.c: In function log_used:
drivers/vhost/vhost.c:1906:1:
warning: the frame size of 1040 bytes is larger than 1024 bytes
Signed-off-by: Li Wang <li.wang@windriver.com>
Link: https://lore.kernel.org/r/1600106889-25013-1-git-send-email-li.wang@windriver.comSigned-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>

drivers/vhost/vhost.c

@@ -1897,7 +1897,7 @@ static int log_write_hva(struct vhost_virtqueue *vq, u64 hva, u64 len)
 
 static int log_used(struct vhost_virtqueue *vq, u64 used_offset, u64 len)
 {
-	struct iovec iov[64];
+	struct iovec *iov = vq->log_iov;
 	int i, ret;
 
 	if (!vq->iotlb)

drivers/vhost/vhost.h

@@ -123,6 +123,7 @@ struct vhost_virtqueue {
 	/* Log write descriptors */
 	void __user *log_base;
 	struct vhost_log *log;
+	struct iovec log_iov[64];
 
 	/* Ring endianness. Defaults to legacy native endianness.
 	 * Set to true when starting a modern virtio device. */