vhost: Remove custom vhost rcu usage

Now, vq->private_data is always accessed under vq mutex. No need to play
the vhost rcu trick.

Signed-off-by: Asias He <asias@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

diff --cc drivers/vhost/net.c
Simple merge

diff --cc drivers/vhost/scsi.c
Simple merge

diff --cc drivers/vhost/test.c
Simple merge

diff --cc drivers/vhost/vhost.h
Simple merge