lockd: Clean up nlmsvc_lookup_host()

Clean up.

Change nlmsvc_lookup_host() to be purpose-built for server-side
nlm_host management.  This replaces the generic nlm_lookup_host()
helper function, just like on the client side.  The lookup logic is
specialized for server host lookups.

The server side cache also gets its own specialized equivalent of the
nlm_release_host() function.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>