nd6: Mark several global callouts MPSAFE

[markj]

Summary

The ND6 code was written pre-SMP, and the use of Giant here is vestigal.
Nothing in the ND6 code depends on it.

Also enter the net epoch in in6_tmpaddrtimer(). It needs to do so to
iterate over the ifnet list safely. This callout runs very
infrequently, so it's not likely to be a problem, but it probably
should also be synchronizing with in6_tmpifadd().

I did not (yet) mark the DAD callout as MPSAFE, it requires further
fixes (but also does not depend on Giant).

[bz]

I always wished the argument was a bool or a '''#define''' with a name rather than 1 but that's unrelated to here.
Looks okay to me.