kernel - Fix not-quite-nonblocking VX lock in allocfreevnode()

kernel - Fix not-quite-nonblocking VX lock in allocfreevnode()

* Introduce LK_NOSPINWAIT to tell lockmgr() to not even spin on the
  spinlock if it can't get it immediately.

* There is a lock order reversal between vfs_spin and vp->v_spinlock
  where vx_lock_nonblock() can deadlock in allocfreevnode().  Normally
  I'd vhold() the vnode but the freelist code is rather fragile and I
  don't want to have to vdrop() later on.  So instead use LK_NOSPINWAIT
  to avoid the situation.

* This is a hack.