kernel - Fix NFS server deadlock
* _cache_cleanneg() was calling cache_zap() with the non-blocking argument
set to FALSE, causing numerous out-of-order locking situations which
could march up the topology and cause a deadlock.
Fixed by calling cache_zap() with the non-blocking argument set to TRUE.
* This was particularly obvious when running a buildworld with a loopback
NFS mount for /usr/obj, which generates lots of negative cache activity,
but theoretically could cause deadlocks even without using NFS.
Reported-by: YONETANI Tomokazu <qhwt.dfly@les.ath.cx>