Constantify lookup key in ifa_ifwith* functions.
Some places in our network stack already have const
arguments (like if_output() routines and LLE functions).

Code using ifa_ifwith (and similar functins) along with
LLE/_output functions is currently bound to use tricks
like __DECONST(). Provide a cleaner way by making sockaddr
lookup key really constant.

MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D3464