kernel - Fix rare race in namecache

* Fix a rare race in _cache_cleanneg() where the ncp being cleaned up is
  resolved during the moment between where _cache_cleanneg() accesses it
  prior to locking and removing it.

* _cache_cleanneg() needed to re-check that the ncp was still on the
  negative cache list.

Reported-by: marino
rc.d/addswap: Load the vn(4) module if not already present.

Reported-by: lentferj