Fix static analysis errors in IPv6 code

[jtl]

Summary

I ran clang's static analyzer over the kernel sources. I've fixed a number of the errors in the IPv6 code.

These run the range from relatively important (could cause a kernel crash) to fairly minor (variables that are set but not used).

Sponsored by: Netflix
MFC after: 2 weeks

[bz]

Can you make a first patch which is just "Remove unused variables"? That way removing the functional changes is a lot easier.
Also when you have to go back in 5 years and wonder what this all was.

[jtl]

At @bz's request, I opened D10940, D10941, D10942, and D10943 for the component parts of this revision.