Btrfs: btrfs_mark_extent_written uses the wrong slot

My test do: fallocate a big file and do write. The file is 512M, but
after file write is done btrfs-debug-tree shows:
item 6 key (257 EXTENT_DATA 0) itemoff 3516 itemsize 53
                extent data disk byte 1103101952 nr 536870912
                extent data offset 0 nr 399634432 ram 536870912
                extent compression 0
Looks like a regression introducted by
6c7d54ac, where we set wrong slot.
Signed-off-by: NShaohua Li <shaohua.li@intel.com>
Acked-by: NYan Zheng <zheng.yan@oracle.com>
Signed-off-by: NChris Mason <chris.mason@oracle.com>

fs/btrfs/file.c

@@ -720,13 +720,15 @@ int btrfs_mark_extent_written(struct btrfs_trans_handle *trans,
 					inode->i_ino, orig_offset);
 		BUG_ON(ret);
 	}
-	fi = btrfs_item_ptr(leaf, path->slots[0],
-			   struct btrfs_file_extent_item);
 	if (del_nr == 0) {
+		fi = btrfs_item_ptr(leaf, path->slots[0],
+			   struct btrfs_file_extent_item);
 		btrfs_set_file_extent_type(leaf, fi,
 					   BTRFS_FILE_EXTENT_REG);
 		btrfs_mark_buffer_dirty(leaf);
 	} else {
+		fi = btrfs_item_ptr(leaf, del_slot - 1,
+			   struct btrfs_file_extent_item);
 		btrfs_set_file_extent_type(leaf, fi,
 					   BTRFS_FILE_EXTENT_REG);
 		btrfs_set_file_extent_num_bytes(leaf, fi,