NVMe: remove QUEUE_FLAG_DEFAULT

No need and queue_set_unlocked don't take a mask anyway.
NVMe: Limit to 4K IOs for LightNVM devices

Currently LightNVM rewrites the logical address, and as LightNVM expects
4K addresses to be handled, this pose a problem for large IO sizes.
Restrict the IO size to 4K until we have a good method for handling
larger IOs.