kernel - Overhaul namecache operations to reduce SMP contention

kernel - Overhaul namecache operations to reduce SMP contention

* Overhaul the namecache code to remove a significant amount of cacheline
  ping-ponging from the namecache paths.  This primarily effects
  multi-socket systems but also improves multi-core single-socket systems.

  Cacheline ping-ponging in the critical path can constrict a multi-core
  system to roughly ~1-2M operations per second running through that path.
  For example, even if looking up different paths or stating different
  files, even something as simple as a non-atomic ++global_counter
  seriously derates performance when it is being executed on all cores at
  once.

  In the simple non-conflicting single-component stat() case, this improves
  performance from ~2.5M/second to ~25M/second on a 4-socket 48-core opteron
  and has a similar improvement on a 2-socket 32-thread xeon, as well as
  significantly improves namecache perf on single-socket multi-core systems.

* Remove the vfs.cache.numcalls and vfs.cache.numchecks debugging counters.
  These global counters caused significant cache ping-ponging and were only
  being used for debugging.

* Implement a poor-man's referenced-structure pcpu cache for struct mount
  and struct namecache.  This allows atomic ops on the ref-count for these
  structures to be avoided in certain critical path cases.  For now limit
  to ncdir and nrdir (nrdir particularly, which is usually the same across
  nearly all processes in the system).  Eventually we will want to expand
  this cache to handle more cases.

  Because we are holding refs persistently, add a bit of infrastructure to
  clear the cache as necessary (e.g. when doing an unmount, for example).

* Shift the 'cachedvnodes' global to a per-cpu accumulator, then roll-up
  the counter back to the global approximately once per second.  The code
  critical paths adjust only the per-cpu accumulator, removing another
  global cache ping-pong from nearly all vnode and nlookup paths.

* The nlookup structure now 'Borrows' the ucred reference from td->td_ucred
  instead of crhold()ing it, removing another global ref/unref from all
  nlookup paths.

* We have a large hash table of spinlocks for nchash, add a little pad
  from 24 to 32 bytes.  Its ok that two spin locks share the same cache
  line (its a huge table), adding the pad cleans up cacheline-crossing
  cases.

* Add a bit of pad to put mount->mnt_refs on its own cache-line verses
  prior fields which are accessed shared.  But don't bother isolating it
  completely.