Btrfs: Fix nodatacow extent lookup

Yan Zheng noticed the offset into the extent was incorrectly being added to the
extent start before trying to find it in the extent allocation tree.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/inode.c

@@ -177,7 +177,6 @@ again:
 		if (bytenr == 0)
 			goto not_found;
 
-		bytenr += btrfs_file_extent_offset(leaf, item);
 		if (btrfs_count_snapshots_in_path(root, path, bytenr) != 1) {
 			goto not_found;
 		}