virtio_pci_modern: introduce helper to map vq notify area

stable inclusion
from stable-v5.13
commit 9e3bb9b7
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I5WXCZ
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9e3bb9b79a7131a088cfffbdcc30e747dad9d090



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This patch factors out the logic of vq notify area mapping. Following
patches will switch to use this common helpers for both virtio_pci
library and virtio-pci vDPA driver.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20210415073147.19331-2-jasowang@redhat.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Eli Cohen <elic@nvidia.com>
Signed-off-by: Pengyuan Zhao <zhaopengyuan@hisilicon.com>