btrfs: remove the dead comment in writepage_delalloc()

When btrfs_run_delalloc_range() failed, we will error out.

But there is a strange comment mentioning that
btrfs_run_delalloc_range() could have returned value >0 to indicate the
IO has already started.

Commit 40f76580 ("Btrfs: split up __extent_writepage to lower stack
usage") introduced the comment, but unfortunately at that time, we were
already using @page_started to indicate that case, and still return 0.

Furthermore, even if that comment was right (which is not), we would
return -EIO if the IO had already started.

By all means the comment is incorrect, just remove the comment along
with the dead check.

Just to be extra safe, add an ASSERT() in btrfs_run_delalloc_range() to
make sure we either return 0 or error, no positive return value.
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_io.c

@@ -3803,13 +3803,7 @@ static noinline_for_stack int writepage_delalloc(struct btrfs_inode *inode,
 		if (ret) {
 			btrfs_page_set_error(inode->root->fs_info, page,
 					     page_offset(page), PAGE_SIZE);
-			/*
-			 * btrfs_run_delalloc_range should return < 0 for error
-			 * but just in case, we use > 0 here meaning the IO is
-			 * started, so we don't want to return > 0 unless
-			 * things are going well.
-			 */
-			return ret < 0 ? ret : -EIO;
+			return ret;
 		}
 		/*
 		 * delalloc_end is already one less than the total length, so

fs/btrfs/inode.c

@@ -1962,6 +1962,7 @@ int btrfs_run_delalloc_range(struct btrfs_inode *inode, struct page *locked_page
 		ret = cow_file_range_async(inode, wbc, locked_page, start, end,
 					   page_started, nr_written);
 	}
+	ASSERT(ret <= 0);
 	if (ret)
 		btrfs_cleanup_ordered_extents(inode, locked_page, start,
 					      end - start + 1);