Revert "vhost: make msg padding explicit"

This reverts commit 93c0d549c4c5a7382ad70de6b86610b7aae57406.

Unfortunately the padding will break 32 bit userspace.
Ouch. Need to add some compat code, revert for now.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>