nvme: Move nvme_freeze/unfreeze_queues to nvme core

Nothing pci specific about them and We'll need them exported
in other transports too.

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>