Deadlock on NFS server

[Matthew Dillon]

    Bingo.  _cache_cleanneg() was calling cache_zap() without specifying
    non-blocking operation.  Since cache_cleanneg() tries to clean up
    ncps in no particular order and out-of-context it must specify
    non-blocking. I have committed a fix.

    Negative ncp entries are seriously exercised by a buildworld due to
    the compiler trying to find #include files.

    I don't think this is related to Francois Tigeot's hammer cleanup
    deadlock report though, or Matthias Schmidt's hammer cleanup / crypto
    deadlock report.  I will continue to try to reproduce those test cases.

						-Matt