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>