nvme: take const cmd pointer in read-only helpers

nvme_is_fabrics() and nvme_is_write() only read struct nvme_command,
so take it by const pointer. This allows callers to pass a const pointer
and communicates that these functions don't modify the command.

Signed-off-by: Caleb Sander <csander@purestorage.com>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Keith Busch <kbusch@kernel.org>