nvme: enable batched completions of passthrough IO

Now that the normal passthrough end_io path doesn't need the request
anymore, we can kill the explicit blk_mq_free_request() and just pass
back RQ_END_IO_FREE instead. This enables the batched completion from
freeing batches of requests at the time.

This brings passthrough IO performance at least on par with bdev based
O_DIRECT with io_uring. With this and batche allocations, peak performance
goes from 110M IOPS to 122M IOPS. For IRQ based, passthrough is now also
about 10% faster than previously, going from ~61M to ~67M IOPS.

Reviewed-by: Anuj Gupta <anuj20.g@samsung.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Co-developed-by: Stefan Roesch <shr@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>