kernel - namecache MPSAFE work

kernel - namecache MPSAFE work

* Most of the MPSAFE coding required for namecache operation.  The MP
  lock still surrounds this code.

* Use a per-bucket spinlock for nchashtbl[] lookups.

* Use a global spinlock for ncneglist.

* Use a global token for nc_parent interlocks.

* Use a per-vnode spinlock (v_spinlock == v_lock.lk_spinlock) to
  manage access to the v_namecache list.

* Recode namecache locks to use atomic_cmpset_ptr() based around
  nc_locktd instead of nc_exlocks.  nc_exlocks is still used to
  track the exclusive lock count.

  NOTE: There may be an issue with the way nc_lockreq is handled.

* Recode cache_hold/drop to use atomic_cmpset_int().

* Carefully code cache_zap() for MPSAFEness.  In particular, multiple
  locks must be held before it can be determined that a namecache
  structure no longer has any referers.  Here is where having the global
  token is really useful.

* cache_fullpath() and vn_fullpath() previously assumed the good graces
  of the MP lock and didn't bother holding refs on the namecache pointers
  they were traversing.  Now they do.

* nlookup*() functions also previously made assumptions with regards
  to holding refs.  Now they properly hold refs.

* struct namecache's nc_flag field is no longer modified outside of
  holding a lock on the structure, so we do not have to resort to
  atomic ops.