vhost: fix error check in vhost_verify_ring_mappings()

Since commit f1f9e6c5 "vhost: adapt vhost_verify_ring_mappings() to
virtio 1 ring layout", we check the mapping of each part (descriptor
table, available ring and used ring) of each virtqueue separately.

The checking of a part is done by the vhost_verify_ring_part_mapping()
function: it returns either 0 on success or a negative errno if the
part cannot be mapped at the same place.

Unfortunately, the vhost_verify_ring_mappings() function checks its
return value the other way round. It means that we either:
- only verify the descriptor table of the first virtqueue, and if it
  is valid we ignore all the other mappings
- or ignore all broken mappings until we reach a valid one

ie, we only raise an error if all mappings are broken, and we consider
all mappings are valid otherwise (false success), which is obviously
wrong.

This patch ensures that vhost_verify_ring_mappings() only returns
success if ALL mappings are okay.

Reported-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
