nvme-pci: simplify nvme_dbbuf_dma_alloc

Move the OACS check and the error checking into nvme_dbbuf_dma_alloc so
that an upcoming second caller doesn't have to duplicate this boilerplate
code.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>