specfs: Drain all I/O operations after last .d_close call.

New kind of I/O reference on specdevs, sd_iocnt.  This could be done
with psref instead; I chose a reference count instead for now because
we already have to take a per-object lock anyway, v_interlock, for
vdead_check, so another atomic is not likely to hurt much more.  We
can always change the mechanism inside spec_io_enter/exit/drain later
on.

Make sure every access to vp->v_rdev or vp->v_specnode and every call
to a devsw operation is protected either:

- by the vnode lock (with vdead_check if we unlocked/relocked),
- by positive sd_opencnt,
- by spec_io_enter/exit, or
- by sd_opencnt management in open/close.

(riastradh)