nvmet: Use direct IO for writes

We're designed to work with high-end devices where
direct IO makes perfect sense. We noticed that we
context switch by scheduling kblockd instead of going
directly to the device without REQ_SYNC for writes.

Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Jens Axboe <axboe@kernel.dk>