engine: fix error handling for e4defrag/falloc

engine: fix error handling for e4defrag/falloc

At the time I've wrote this code i don't quite understand difference
between td->error and io_u->error. It is appeared that engine should
not have to explicitly assign td->error. Just initialize io_u->error and
backed will do proper handling.

Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>