nvme: introduce nvme_start_request

In preparation for nvme-multipath IO stats accounting, we want the
accounting to happen in a centralized place. The request completion
is already centralized, but we need a common helper to request I/O
start.

Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Hannes Reinecke <hare@suse.de>