ND_IFINFO() cannot be NULL, use inline read-only idiom for clarity

ND_IFINFO() cannot be NULL, use inline read-only idiom for clarity

ND_IFINFO() always points at a valid struct nd_ifinfo;  ND6_LLINFO_DELAY
checks for NULL, while other cases in nd6_llinfo_timer() dereference it
unconditionally.

Inline all three per-case read-only usages rather than having one hoisted
*ndi pointer which could be used to write.

nd6_nbr.c already uses this `ND_IFINFO(ifp)->retrans' idiom which makes it
immediately clear that data is only read.

OK bluhm