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/compression.c b/fs/btrfs/compression.c
index 15b9408..b9c5650 100644
--- a/fs/btrfs/compression.c
+++ b/fs/btrfs/compression.c
@@ -106,7 +106,6 @@
 				 u64 disk_start)
 {
 	int ret;
-	struct btrfs_root *root = BTRFS_I(inode)->root;
 	struct page *page;
 	unsigned long i;
 	char *kaddr;
@@ -121,7 +120,7 @@
 		csum = ~(u32)0;
 
 		kaddr = kmap_atomic(page);
-		csum = btrfs_csum_data(root, kaddr, csum, PAGE_CACHE_SIZE);
+		csum = btrfs_csum_data(kaddr, csum, PAGE_CACHE_SIZE);
 		btrfs_csum_final(csum, (char *)&csum);
 		kunmap_atomic(kaddr);