NLM: Convert nlm_lookup_host() to use a single argument

The nlm_lookup_host() function already has a large number of arguments,
and I'm about to add a few more.  As a clean up, convert the function
to use a single data structure argument.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>