vhost: Fix the calculation in vhost_overflow()

vhost: Fix the calculation in vhost_overflow()

[ Upstream commit f7ad318ea0ad58ebe0e595e59aed270bb643b29b ]

This fixes the incorrect calculation for integer overflow
when the last address of iova range is 0xffffffff.

Fixes: 9ae11f321f6b ("vhost: detect 32 bit integer wrap around")
Reported-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20210728130756.97-2-xieyongji@bytedance.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>