virtio_balloon: return the amount of freed memory from leak_balloon()

This value would be useful in the next patch to provide the amount of
the freed memory for OOM killer.
Signed-off-by: Raushaniya Maksudova <rmaksudova@parallels.com>
Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Rusty Russell <rusty@rustcorp.com.au>
CC: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

drivers/virtio/virtio_balloon.c

@@ -168,8 +168,9 @@ static void release_pages_by_pfn(const u32 pfns[], unsigned int num)
 	}
 }
 
-static void leak_balloon(struct virtio_balloon *vb, size_t num)
+static unsigned leak_balloon(struct virtio_balloon *vb, size_t num)
 {
+	unsigned num_freed_pages;
 	struct page *page;
 	struct balloon_dev_info *vb_dev_info = &vb->vb_dev_info;
 
@@ -186,6 +187,7 @@ static void leak_balloon(struct virtio_balloon *vb, size_t num)
 		vb->num_pages -= VIRTIO_BALLOON_PAGES_PER_PAGE;
 	}
 
+	num_freed_pages = vb->num_pfns;
 	/*
 	 * Note that if
 	 * virtio_has_feature(vdev, VIRTIO_BALLOON_F_MUST_TELL_HOST);
@@ -195,6 +197,7 @@ static void leak_balloon(struct virtio_balloon *vb, size_t num)
 		tell_host(vb, vb->deflate_vq);
 	mutex_unlock(&vb->balloon_lock);
 	release_pages_by_pfn(vb->pfns, vb->num_pfns);
+	return num_freed_pages;
 }
 
 static inline void update_stat(struct virtio_balloon *vb, int idx,