commit e4af376d04b0(drivers: hv: switch to use mb() instead of smp_mb()),
the adjustment mistakenly dropped the change in hv_ringbuffer_read,
so add it.
Signed-off-by: Qiang Huang <h.huangqiang@huawei.com> Signed-off-by: Ben Hutchings <ben@decadent.org.uk>