rbd: Fix ceph_snap_context size calculation

ceph_snap_context->snaps is an u64 array

Signed-off-by: Zheng Yan <zheng.z.yan@intel.com>
Reviewed-by: Alex Elder <elder@inktank.com>
(cherry picked from commit f9f9a19)

drivers/block/rbd.c

@@ -499,7 +499,7 @@ static int rbd_header_from_disk(struct rbd_image_header *header,
 			 / sizeof (*ondisk))
 		return -EINVAL;
 	header->snapc = kmalloc(sizeof(struct ceph_snap_context) +
-				snap_count * sizeof (*ondisk),
+				snap_count * sizeof(u64),
 				gfp_flags);
 	if (!header->snapc)
 		return -ENOMEM;