Combine route_cache() and rtalloc_mpath() in new route_mpath().

Combine route_cache() and rtalloc_mpath() in new route_mpath().

Fill and check the cache and call rtalloc_mpath() together.  Then
the caller of route_mpath() does not have to care about the uint32_t
*src pointer and just pass struct in_addr.  All the conversions are
done inside the functions.  ro->ro_rt is either valid or NULL.  Note
that some places have a stricter rtisvalid() now compared to the
previous NULL check.

OK claudio@