ieee1394: sbp2: prevent rare deadlock in shutdown

Scsi_remove_device() may go into uninterruptible sleep if blocked.
Therefore sbp2_remove() unblocks the Scsi_Host before the device is
requested to be removed.  But there could be another 1394 bus reset
after that which would block the host again.  The 1394 subsystem won't
call sbp2_update() concurrently to sbp2_remove(), which is why there is
no chance for sbp2_remove() to be unblocked by sbp2_update().

The fix is to tell sbp2's bus reset handler when a device is to be shut
down so that it skips scsi_block_requests() on that host.  As before,
any new commands after a reset without reconnect will be failed quickly
by sbp2scsi_queuecommand().

In the long term, means to go without scsi_block_requests() should be
found.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>