cache: create a dedicate struct for negative entries

.. and stuff if into the unused target vnode field

This gets rid of concurrent nc_flag modifications racing with the
shrinker and consequently fixes a bug where such a change could have
been missed when cache_ncp_invalidate was being issued..

Reported by: zeising
Tested by: pho, zeising
Fixes: r362828 ("cache: lockless forward lookup with smr")