Rewrite in_ouraddr() to not use ``rt_ifa'' since it is not obvious that

Rewrite in_ouraddr() to not use ``rt_ifa'' since it is not obvious that
the lifetime of an ``ifa'' is tied to a route entry, so it might no
longer be valid after calling rtfree(9).

While here put a KERNEL_LOCK() around the per-ifp address list iteration.

ok bluhm@