xen/gntdev: reduce copy batch size to 16

IOCTL_GNTDEV_GRANT_COPY batches copy operations to reduce the number
of hypercalls.  The stack is used to avoid a memory allocation in a
hot path. However, a batch size of 24 requires more than 1024 bytes of
stack which in some configurations causes a compiler warning.

    xen/gntdev.c: In function ‘gntdev_ioctl_grant_copy’:
    xen/gntdev.c:949:1: warning: the frame size of 1248 bytes is
    larger than 1024 bytes [-Wframe-larger-than=]

This is a harmless warning as there is still plenty of stack spare,
but people keep trying to "fix" it.  Reduce the batch size to 16 to
reduce stack usage to less than 1024 bytes.  This should have minimal
impact on performance.
Signed-off-by: David Vrabel <david.vrabel@citrix.com>

drivers/xen/gntdev.c

@@ -748,7 +748,7 @@ static long gntdev_ioctl_notify(struct gntdev_priv *priv, void __user *u)
 	return rc;
 }
 
-#define GNTDEV_COPY_BATCH 24
+#define GNTDEV_COPY_BATCH 16
 
 struct gntdev_copy_batch {
 	struct gnttab_copy ops[GNTDEV_COPY_BATCH];