Btrfs: fix comment in btrfs_page_mkwrite

The comment about "page_mkwrite gets called every time the page is
dirtied" in btrfs_page_mkwrite is not correct, it only gets called the
first time the page gets dirtied after the page faults in.

However, we don't need to touch the code because it works well, although
the proper logic is to check if delalloc bits has been set and if so, go
free reserved space, if not, set the delalloc bits for dirty page range.

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -9060,11 +9060,11 @@ again:
 	}
 
 	/*
-	 * XXX - page_mkwrite gets called every time the page is dirtied, even
-	 * if it was already dirty, so for space accounting reasons we need to
-	 * clear any delalloc bits for the range we are fixing to save.  There
-	 * is probably a better way to do this, but for now keep consistent with
-	 * prepare_pages in the normal write path.
+	 * page_mkwrite gets called when the page is firstly dirtied after it's
+	 * faulted in, but write(2) could also dirty a page and set delalloc
+	 * bits, thus in this case for space account reason, we still need to
+	 * clear any delalloc bits within this page range since we have to
+	 * reserve data&meta space before lock_page() (see above comments).
 	 */
 	clear_extent_bit(&BTRFS_I(inode)->io_tree, page_start, end,
 			  EXTENT_DIRTY | EXTENT_DELALLOC |