kernel - Fix NFS stall and likely also x86-64 seg-fault issue

kernel - Fix NFS stall and likely also x86-64 seg-fault issue

* The nfs node hash code was not MPSAFE due to its use of a global
  hash table.  This could lead to a stall condition due to the
  global hash table losing track of its manual lock.  For now use a
  token to interlock hash table access and replace the manual lock
  with a lockmgr lock.

* This appears to have also fixed the long-standing random seg-fault
  issue with x86-64, where a buildworld loop would seg-fault every once
  in a while for no apparent reason.  /usr/src on my test box has always
  been NFS mounted but was never considered a possible contributor to
  the problem.

  If NFS loses track of related nfs nodes or races operations on underlying
  vnodes while expecting their storage to remain stable it is possible
  for a random vnode to become corrupt.  How this could translate to a
  completely random seg-fault is not entirely understood but I surmise
  the unrelated cc1's binary mmap could become corrupt.

Reported-by: Thomas Nikolajsen <thomas.nikolajsen@mail.dk> (nfs issue)