rdma: use resp.len after validation in qemu_rdma_registration_stop

resp.len is given from remote host. So should be validated before use.
Otherwise memcpy can access beyond the buffer.

Cc: Michael R. Hines <mrhines@us.ibm.com>
Reviewed-by: Orit Wasserman <owasserm@redhat.com>
Reviewed-by: Michael R. Hines <mrhines@us.ibm.com>
Signed-off-by: Isaku Yamahata <yamahata@private.email.ne.jp>
Signed-off-by: Michael R. Hines <mrhines@us.ibm.com>
Message-id: 1376078746-24948-2-git-send-email-mrhines@linux.vnet.ibm.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
