uwb: properly check kthread_run return value

uwb: properly check kthread_run return value

uwbd_start() calls kthread_run() and checks that the return value is
not NULL. But the return value is not NULL in case kthread_run() fails,
it takes the form of ERR_PTR(-EINTR).

Use IS_ERR() instead.

Also add a check to uwbd_stop().

Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit bbf26183b7a6236ba602f4d6a2f7cade35bba043)

Orabug: 27206874
CVE: CVE-2017-16526

Signed-off-by: Tim Tianyang Chen <tianyang.chen@oracle.com>
Reviewed-by: Reviewed-by: Jack Vogel <jack.vogel@oracle.com>