nvme: move nvme_multi_css into nvme.h

Preparatory patch in order to reuse nvme_multi_css in the nvme target
code.

Signed-off-by: Adam Manzanares <a.manzanares@samsung.com>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>