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

stable inclusion
from stable-v5.10.198
commit 71357c751fb25c9c7ced051419dbddd02250b91d
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I987V5

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=71357c751fb25c9c7ced051419dbddd02250b91d



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[ 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>
Signed-off-by: sanglipeng <sanglipeng1@jd.com>