nvme-pci: cleanup SQ allocation a bit

Use a bit flag to mark if the SQ was allocated from the CMB, and clean
up the surrounding code a bit.

Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>