usblp: poison URBs upon disconnect

syzkaller reported an URB that should have been killed to be active.
We do not understand it, but this should fix the issue if it is real.

Signed-off-by: Oliver Neukum <oneukum@suse.com>
Reported-by:  <syzbot+be5b5f86a162a6c281e6@syzkaller.appspotmail.com>
Link: https://lore.kernel.org/r/20200507085806.5793-1-oneukum@suse.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>