Btrfs: cleanup unused arguments of btrfs_csum_data

Argument 'root' is no more used in btrfs_csum_data().

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
diff --git a/fs/btrfs/file-item.c b/fs/btrfs/file-item.c
index c4628a2..bc3c924 100644
--- a/fs/btrfs/file-item.c
+++ b/fs/btrfs/file-item.c
@@ -484,8 +484,7 @@
 
 		data = kmap_atomic(bvec->bv_page);
 		sector_sum->sum = ~(u32)0;
-		sector_sum->sum = btrfs_csum_data(root,
-						  data + bvec->bv_offset,
+		sector_sum->sum = btrfs_csum_data(data + bvec->bv_offset,
 						  sector_sum->sum,
 						  bvec->bv_len);
 		kunmap_atomic(data);