vhost_verify_ring_mappings leaks mappings on error.
Fix this up.

Cc: qemu-stable@nongnu.org
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

(cherry picked from commit 8617343f)
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>