fix invalid bit range when allocate ipv6 address

[houstar]

Fixes #2296

Signed-off-by: Leno Hou [email protected]

[arkodg]

@houstar thanks for working on this, can you please add some test-cases as well

[arkodg]

also TestValidRemoteDriver should be fixed in master, all tests should hopefully pass if you re-run tests

[akerouanton]

Thanks @houstar for submitting this PR. However, time as passed since it was originally submitted and I'm going to close it for the following reasons:

1. This repo has been moved to https://github.com/moby/moby/tree/master/libnetwork. Any new work should happen there, and backported in this repo iff required ;
2. As stated by @fabianfreyer, part of the problem is the use of a uint64 for doing computation on 128 bits. This is getting addressed in moby/moby#43033, although while working on that PR we uncovered other issues that require us to switch to some more advanced data structures to make sure two subnets don't overlap ;