nvme-multipath: fix io accounting on failover

There are io stats accounting that needs to be handled, so don't call
blk_mq_end_request() directly. Use the existing nvme_end_req() helper
that already handles everything.

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>
Signed-off-by: Keith Busch <kbusch@kernel.org>