nvme: change __nvme_submit_sync_cmd() calling conventions

Combine the two arguments 'flags' and 'at_head' from __nvme_submit_sync_cmd()
into a single 'flags' argument and use function-specific values to indicate
what should be set within the function.

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