IP address limits might not suitable for IPv6

[qin-nz]

Hi all,
In this code, geth restrict only 2 IP address is allowed in the same subnet (/24) to prevent from attack of the eclipse.

go-ethereum/p2p/discover/table.go

Lines 52 to 53 in 9e0f934

	// IP address limits.
	bucketIPLimit, bucketSubnet = 2, 24 // at most 2 addresses from the same /24

In IPv4, it's ok. But in IPv6, /24 is quite a big subnet.
For example,

• China Telecom (Tire 1 ISP in China, with hundred million users ) only have a /20 subnet (=16*/24) (see https://bgp.he.net/ip/240e:: ).
• Alibaba/Aliyun (No.1 Public Cloud Provider in China) only have a /22 subnet (=4*/24) (see https://bgp.he.net/net/2408:4000::/22 ).

I think good limit should be:

• 2 ips for /48 subnet

Reasons:
/24 in IPv4 is the minium length in GLOBAL routing table.
/48 in IPv6 is the minium length in GLOBAL routing table.

For higher securtiy, the follwing limit may also be considerate:

• 8 ips for /32 subnet

[fjl]

Thank you for bringing this up. I will investigate how to improve IPv6 limits very soon.