vhost: allow batching hint without size

mainline inclusion
from mainline-v5.17
commit 95932ab2
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I5WXCZ
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=95932ab2ea07b79cdb33121e2f40ccda9e6a73b5



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Commit e2ae38cf ("vhost: fix hung thread due to erroneous iotlb
entries") tries to reject the IOTLB message whose size is zero. But
the size is not necessarily meaningful, one example is the batching
hint, so the commit breaks that.

Fixing this be reject zero size message only if the message is used to
update/invalidate the IOTLB.

Fixes: e2ae38cf ("vhost: fix hung thread due to erroneous iotlb entries")
Reported-by: Eli Cohen <elic@nvidia.com>
Cc: Anirudh Rayabharam <mail@anirudhrb.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20220310075211.4801-1-jasowang@redhat.com


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