usb-storage: implement "soft" unbinding

This patch (as1092) implements "soft" unbinding for usb-storage.  When
the disconnect routine is called, all commands and reset delays are
allowed to complete normally until after scsi_remove_host() returns.
This means that the commands needed for an orderly shutdown will be
sent through to the device.

Unlike before, the driver will now execute every command that it
accepts.  Hence there's no need for special code to catch unexecuted
commands and fail them.

The new sequence of events when disconnect runs goes as follows:

	If the device is truly unplugged, set the DISCONNECTING
	flag so we won't try to access it any more.

	If the SCSI-scanning thread hasn't started up yet, prevent
	it from doing anything by setting the new DONT_SCAN flag.
	Then wake it up and wait for it to terminate.

	Remove the SCSI host.  This unbinds the upper-level drivers,
	doing an orderly shutdown.  Commands sent to quiesce the
	device will be transmitted normally, unless the device is
	unplugged.

	Set the DISCONNECTING flag so that we won't accept any new
	commands that might get submitted (there aren't supposed to be
	any) and we won't try to access the device for resets.

	Tell the control thread to exit by waking it up with no
	pending command, and wait for it to terminate.

	Go on to do all the other normal stuff: releasing resources,
	freeing memory, and so on.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>