usbfs: fix race between open and unregister

This patch (as1106) fixes a race between opening and unregistering
device files in usbfs.  The current code drops its reference to the
device and then reacquires it, ignoring the possibility that the
device structure might have been removed in the meantime.  It also
doesn't check whether the device is already in the NOTATTACHED state
when the file is opened.

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