ext4: dirty page has always buffers attached

ext4_writepage(), write_cache_pages_da(), and mpage_da_submit_io()
doesn't have to deal with the case when page doesn't have buffers. We
attach buffers to a page in ->write_begin() and ->page_mkwrite() which
covers all places where a page can become dirty.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>