vhost: don't hold onto file pointer for VHOST_SET_VRING_CALL

We already hold a reference to the eventfd_ctx, which is sufficient;
there's no need to hold a reference to the struct file as well.  So get
rid of vhost_virtqueue->call.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Jason Wang <jasowang@redhat.com>

drivers/vhost/vhost.h

@@ -92,7 +92,6 @@ struct vhost_virtqueue {
 	struct vring_used __user *used;
 	const struct vhost_umem_node *meta_iotlb[VHOST_NUM_ADDRS];
 	struct file *kick;
-	struct file *call;
 	struct file *error;
 	struct eventfd_ctx *call_ctx;
 	struct eventfd_ctx *error_ctx;