If there is no journal, ext4_should_writeback_data() should return
TRUE. This will fix ext4_set_aops() to set ext4_da_ops in the case of
delayed allocation; otherwise ext4_journaled_aops gets used by
default, which doesn't handle delayed allocation properly.
The advantage of using ext4_should_writeback_data() approach is that
it should handle nobh better as well.
Thanks to Curt Wohlgemuth for investigating this problem, and Aneesh
Kumar for suggesting this approach.