nvme-multipath: set QUEUE_FLAG_NOWAIT

The nvme multipathing code just dispatches bios to one of the blk-mq
based paths and never blocks on its own, so set QUEUE_FLAG_NOWAIT
to support REQ_NOWAIT bios.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>