kernel - Deal with VOP_NRENAME races

kernel - Deal with VOP_NRENAME races

* VOP_NRENAME() as implemented by the kernel can race any number of
  ways, including deadlocking, allowing duplicate entries, and panicing
  tmpfs.  It typically requires a heavy test load to replicate this but
  a dsynth build triggered the issue at least once.

  Other recently reported tmpfs issues with log file handling might also
  be effected.

* A per-mount (semi-global) lock is now obtained whenever a directory
  is renamed.  This helps deal with numerous MP races that can cause
  lock order reversals.

  Loosely taken from netbsd and linux (mjg brought me up to speed on
  this).  Renaming directories is fraught with issues and this fix,
  while somewhat brutish, is fine.  Directories are very rarely renamed
  at a high rate.

* kern_rename() now proactively locks all four elements of a rename
  operation (source_dir, source_file, dest_dir, dest_file) instead of
  only two.

* The new locking function, cache_lock4_tondlocked(), takes no chances
  on lock order reversals and will use a (currently brute-force)
  non-blocking and lock cycling algorithm.  Probably needs some work.

* Fix a bug in cache_nlookup() related to reusing DESTROYED entries
  in the hash table.  This algorithm tried to reuse the entries while
  maintaining shared locks, since only the entries need to be manipulate
  to reuse them.  However, this resulted in lookup races which could
  cause duplicate entries.  The duplicate entries then triggered
  assertions in TMPFS.

* nlookup now tries a little harder and will retry if the parent of an
  element is flagged DESTROYED after its lock was released.  DESTROYED
  elements are not necessarily temporary events as an operation can wind
  up running in a deleted directory and must properly fail under those
  conditions.

* Use krateprintf() to reduce debug output related to rename race
  reporting.

* Revamp nfsrv_rename() as well (requires more testing).

* Allow nfs_namei() to be called in a loop for retry purposes if
  desired.  It now detects that the nd structure is initialized
  from a prior run and won't try to re-parse the mbuf (needs testing).

Reported-by: zrj, mjg