Btrfs: Properly clear dirty and delalloc extent bits while preparing the file for write

Yan Zheng noticed that we don't clear the extent state tree dirty and delalloc
bits when we clear the dirty bits on the page during file write.

This leads to csum errors later on.
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/file.c

@@ -762,6 +762,13 @@ static int prepare_pages(struct btrfs_root *root, struct file *file,
 		set_page_extent_mapped(pages[i]);
 		WARN_ON(!PageLocked(pages[i]));
 	}
+	if (start_pos < inode->i_size) {
+		u64 last_pos;
+		last_pos = (index + num_pages) << PAGE_CACHE_SHIFT;
+		clear_extent_bits(&BTRFS_I(inode)->io_tree, start_pos,
+				  last_pos - 1, EXTENT_DIRTY | EXTENT_DELALLOC,
+				  GFP_NOFS);
+	}
 	return 0;
 }