grml-sniff: disable IPv6 to avoid neighbor solicitation/multicast traffic

grml-sniff: disable IPv6 to avoid neighbor solicitation/multicast traffic

With IPv6 enabled the devices leak IPv6 traffic because
of the way IPv6 works™:

| 16:24:20.074982 IP6 (hlim 1, next-header Options (0) payload length: 36) :: > ff02::16: HBH (rtalert: 0x0000) (padn) [icmp6 sum ok] ICMP6, multicast listener report v2, 1 group record(s) [gaddr [...] to_ex { }]
| 16:24:20.319025 IP6 (hlim 255, next-header ICMPv6 (58) payload length: 24) :: > ff02::1:ffab:3f90: [icmp6 sum ok] ICMP6, neighbor solicitation, length 24, who has [...]
| 16:24:21.319027 IP6 (hlim 255, next-header ICMPv6 (58) payload length: 16) [...] > ip6-allrouters: [icmp6 sum ok] ICMP6, router solicitation, length 16
| [...]
| 16:24:23.619025 IP6 (hlim 1, next-header Options (0) payload length: 36) [...] > ff02::16: HBH (rtalert: 0x0000) (padn) [icmp6 sum ok] ICMP6, multicast listener report v2, 1 group record(s) [gaddr ff02::1:ffab:3f90 to_ex { }]
| 16:24:25.330982 IP6 (hlim 255, next-header ICMPv6 (58) payload length: 16) [...] > ip6-allrouters: [icmp6 sum ok] ICMP6, router solicitation, length 16
|           source link-address option (1), length 8 (1): [...]
|             0x0000:  001d 92ab 3f90
| 16:24:29.338972 IP6 (hlim 255, next-header ICMPv6 (58) payload length: 16) [...] > ip6-allrouters: [icmp6 sum ok] ICMP6, router solicitation, length 16
|           source link-address option (1), length 8 (1): [...]
|             0x0000:  001d 92ab 3f90

So when starting the sniffing setup disable IPv6 and
when stopping the sniffing setup re-enable it again.