kernel - fine-grained namecache and partial vnode MPSAFE work

kernel - fine-grained namecache and partial vnode MPSAFE work

			Namecache subsystem

* All vnode->v_flag modifications now use vsetflags() and vclrflags().
  Because some flags are set and cleared by vhold()/vdrop() which
  do not require any locks to be held, all modifications must use atomic
  ops.

* Clean up and revamp the namecache MPSAFE work.  Namecache operations now
  use a fine-grained MPSAFE locking model which loosely follows these
  rules:

  - lock ordering is child to parent.  e.g. lock file, then lock parent
    directory.  This allows resolver recursions up the parent directory
    chain.

  - Downward-traversing namecache invalidations and path lookups will
    unlock the parent (but leave it referenced) before attempting to
    lock the child.

  - Namecache hash table lookups utilize a per-bucket spinlock.

  - vnode locks may be acquired while holding namecache locks but not
    vise-versa.  VNodes are not destroyed until all namecache references
    go away, but can enter reclamation.  Namecache lookups detect the case
    and re-resolve to overcome the race.  Namecache entries are not
    destroyed while referenced.

* Remove vfs_token, the namecache MPSAFE model is now totally fine-grained.

* Revamp namecache locking primitves (cache_lock/cache_unlock and
  friends).  Use atomic ops and nc_exlocks instead of nc_locktd and
  build-in a request flag.  This solves busy/tsleep races between lock
  holder and lock requester.

* Revamp namecache parent/child linkages.  Instead of using vfs_token to
  lock such operations we simply lock both child and parent namecache
  entries.  Hash table operations are also fully integrated with the
  parent/child linking operations.

* The vnode->v_namecache list is locked via vnode->v_spinlock, which
  is actually vnode->v_lock.lk_spinlock.

* Revamp cache_vref() and cache_vget().  The passed namecache entry must
  be referenced and locked.  Internals are simplified.

* Fix a deadlock by moving the call to _cache_hysteresis() to a
  place where the current thread otherwise does not hold any locked
  ncp's.

* Revamp nlookup() to follow the new namecache locking rules.

* Fix a number of places, e.g. in vfs/nfs/nfs_subs.c, where ncp->nc_parent
  or ncp->nc_vp was being accessed with an unlocked ncp.  nc_parent
  and nc_vp accesses are only valid if the ncp is locked.

* Add the vfs.cache_mpsafe sysctl, which defaults to 0.  This may be set
  to 1 to enable MPSAFE namecache operations for [l,f]stat() and open()
  system calls (for the moment).

			VFS/VNODE subsystem

* Use a global spinlock for now called vfs_spin to manage vnode_free_list.
  Use vnode->v_spinlock (and vfs_spin) to manage vhold/vdrop ops and
  to interlock v_auxrefs tests against vnode terminations.

* Integrate per-mount mnt_token and (for now) the MP lock into VOP_*()
  and VFS_*() operations.  This allows the MP lock to be shifted further
  inward from the system calls, but we don't do it quite yet.

* HAMMER: VOP_GETATTR, VOP_READ, and VOP_INACTIVE are now MPSAFE.  The
  corresponding sysctls have been removed.

* FIFOFS: Needed some MPSAFE work in order to allow HAMMER to make things
  MPSAFE above, since HAMMER forwards vops for in-filesystem fifos to
  fifofs.

* Add some debugging kprintf()s when certain MP races are averted, for
  testing only.

				MISC

* Add some assertions to the VM system.

* Document existing and newly MPSAFE code.