There are situations when nodes vanish from the bus and come back in
quickly thereafter:
  - When certain bus-powered hubs are plugged in,
  - when certain disk enclosures are switched from self-power to bus
    power or vice versa and break the daisy chain during the transition,
  - when the user plugs a cable out and quickly plugs it back in, e.g.
    to reorder a daisy chain (works on Mac OS X if done quickly enough),
  - when certain hubs temporarily malfunction during high bus traffic.

The ieee1394 driver's nodemgr already contained a function to set
vanished nodes aside into "limbo"; i.e. they wouldn't actually be
deleted right away.  (In fact, only unloading the driver or writing into
an obscure sysfs attribute would delete them eventually.)  If nodes
reappeared later, they would be resurrected out of limbo.

Moving nodes into and out of limbo was accompanied with calling the
.suspend() and .resume() driver methods of the drivers which were bound
to a respective node's unit directories.  Not only is this somewhat
strange due to the intended use of these driver methods for power
management, also the sbp2 driver in particular does not implement
.suspend() and .resume().  Hence sbp2 would be disconnected from devices
in situations as listed above.

We now:
  - leave drivers bound when nodes go into limbo,
  - call the drivers' .update() when nodes come out of limbo,
  - automatically delete in-limbo nodes 3 seconds after the last
    bus reset and bus rescan.
  - Because of the automatic removal, the now obsolete bus attribute
    /sys/bus/ieee1394/destroy_node is removed.

This especially lets sbp2 survive brief disconnections.  You can for
example yank a disk's cable and plug it back in while reading the
respective disk with dd, but dd will happily continue as if nothing
happened.
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>