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.

A previous version of this diff was backed out.  There was an
additional rtisvalid() in rtalloc_mpath() that prevented packet
output via interfaces that were not up.  Now the route in the cache
has to be valid, but after new lookup, rtalloc_mpath() may return
invalid routes.  This generates less errors in userland an preserves
existing behavior.

OK sashan@