USB: usblp: fix a hang in poll() if disconnected

USB: usblp: fix a hang in poll() if disconnected

Apparently an application that opens a device and calls select()
on it, will hang if the decice is disconnected. It's a little
surprising that we had this bug for 15 years, but apparently
nobody ever uses select() with a printer: only write() and read(),
and those work fine. Well, you can also select() with a timeout.

The fix is modeled after devio.c. A few other drivers check the
condition first, then do not add the wait queue in case the
device is disconnected. We doubt that's completely race-free.
So, this patch adds the process first, then locks properly
and checks for the disconnect.

Reviewed-by: Zqiang <qiang.zhang@windriver.com>
Signed-off-by: Pete Zaitcev <zaitcev@redhat.com>
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20210303221053.1cf3313e@suzdal.zaitcev.lan
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>