virtio: handle > 2 billion page balloon targets

If the host asks for a huge target towards_target() can overflow, and
we up oops as we try to release more pages than we have.  The simple
fix is to use a 64-bit value.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
---
 drivers/virtio/virtio_balloon.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff -r fd0c80dbbd95 drivers/virtio/virtio_balloon.c
--- a/drivers/virtio/virtio_balloon.c	Tue Mar 11 09:21:00 2008 +1100
+++ b/drivers/virtio/virtio_balloon.c	Tue Mar 11 11:25:52 2008 +1100
@@ -152,7 +152,7 @@ static void virtballoon_changed(struct v
 	wake_up(&vb->config_change);
 }
 
-static inline int towards_target(struct virtio_balloon *vb)
+static inline s64 towards_target(struct virtio_balloon *vb)
 {
 	u32 v;
 	__virtio_config_val(vb->vdev,
@@ -176,7 +183,7 @@ static int balloon(void *_vballoon)
 
 	set_freezable();
 	while (!kthread_should_stop()) {
-		int diff;
+		s64 diff;
 
 		try_to_freeze();
 		wait_event_interruptible(vb->config_change,