Clean up: The contents of NL_ADDR are fixed: they are always the IPv4
loopback address. Some time ago, the use of NL_ADDR() was stubbed out
of the NLM downcall forward path, replaced with a constant IPv4
loopback address.
Stub it out of the reply path as well, and then remove NL_ADDR
entirely.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>