nvme-pci: refactor nvme_pci_use_sgls

Move the average segment size into a separate helper, and return a
tristate to distinguish the case where can use SGL vs where we have to
use SGLs.  This will allow the simplify the code and make more efficient
decisions in follow on changes.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Link: https://lore.kernel.org/r/20250625113531.522027-4-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>