nvme-pci: factor the iod mempool creation into a helper

[ Upstream commit 081a7d95 ]

Add a helper to create the iod mempool.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Tested-by Gerd Bayer <gbayer@linxu.ibm.com>
Stable-dep-of: dad651b2 ("nvme-pci: do not set the NUMA node of device if it has none")
Signed-off-by: Sasha Levin <sashal@kernel.org>