nvme: fix multipath batched completion accounting

Batched completions were missing the io stats accounting and bio trace
events. Move the common code to a helper and call it from the batched
and non-batched functions.

Fixes: d4d957b5 ("nvme-multipath: support io stats on the mpath device")
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>