If DHCP is configured for IPv4 and IPv6, failing to obtain an IPv6 IP
does not roll back the allocation made for IPv4. This patch rolls back
any changes made in case of failure, so that IP allocation is actually
atomic.
Signed-off-by: Stefan Hanreich <s.hanreich@proxmox.com> Reviewed-by: Stefan Lendl <s.lendl@proxmox.com> Tested-by: Stefan Lendl <s.lendl@proxmox.com> Signed-off-by: Stefan Lendl <s.lendl@proxmox.com>