Assert that rtrequest1(9) is always called at IPL_SOFTNET instead of

Assert that rtrequest1(9) is always called at IPL_SOFTNET instead of
doing a spl dance.

This matters for RTM_DELETE because a deleted route entry is always
returned without having its reference counter increased when an `rtp'
argument is given.  In this case, we do not want to call splx() that
might free this route.

Inputs from millert@, ok blambert@, mikeb@