usblp: fix race between disconnect() and read()

read() needs to check whether the device has been
disconnected before it tries to talk to the device.

Signed-off-by: Oliver Neukum <oneukum@suse.com>
Reported-by:  <syzbot+be5b5f86a162a6c281e6@syzkaller.appspotmail.com>
Link: https://lore.kernel.org/r/20200917103427.15740-1-oneukum@suse.com


Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>