Use SYSREF to reference count struct vnode. v_usecount is now

Use SYSREF to reference count struct vnode.  v_usecount is now
v_sysref(.refcnt).  v_holdcnt is now v_auxrefs.  SYSREF's termination state
(using a negative reference count from -0x40000000+) now places the vnode in
a VCACHED or VFREE state and deactivates it.  The vnode is now assigned a
64 bit unique id via SYSREF.

vhold() (which manipulates v_auxrefs) no longer reactivates a vnode and
is explicitly used only to track references from auxillary structures
and references to prevent premature destruction of the vnode.  vdrop()
will now only move a vnode from VCACHED to VFREE on the 1->0 transition
of v_auxrefs if the vnode is in a termination state.

vref() will now panic if used on a vnode in a termination state.  vget()
must now be used to explicitly reactivate a vnode.  These requirements
existed before but are now explicitly asserted.

vlrureclaim() and allocvnode() should now interact a bit better.  In
particular, vlrureclaim() will do a better job of finding vnodes to flush
and transition from VCACHED to VFREE, and allocvnode() will do a better
job finding vnodes to reuse without getting blocked by a flush.

allocvnode now uses a real VX lock to sequence vnodes into VRECLAIMED.  All
vnode special state processing now uses a VX lock.

Vnodes are now able to be slowly returned to the memory pool when
kern.maxvnodes is reduced at run time.

Various initialization elements have been moved to CTOR/DTOR and are
no longer in the critical path, improving performance.  However, since
SYSREF uses atomic_cmpset_int() (aka cmpxchgl), which reduces performance
somewhat, overall performance tends to be about the same.