dm log writes: fix check of kthread_run() return value

The kthread_run() function returns either a valid task_struct or
ERR_PTR() value, check for NULL is invalid.  This change fixes potential
for oops, e.g. in OOM situation.
Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Cc: stable@vger.kernel.org

drivers/md/dm-log-writes.c

@@ -459,9 +459,9 @@ static int log_writes_ctr(struct dm_target *ti, unsigned int argc, char **argv)
 		goto bad;
 	}
 
-	ret = -EINVAL;
 	lc->log_kthread = kthread_run(log_writes_kthread, lc, "log-write");
-	if (!lc->log_kthread) {
+	if (IS_ERR(lc->log_kthread)) {
+		ret = PTR_ERR(lc->log_kthread);
 		ti->error = "Couldn't alloc kthread";
 		dm_put_device(ti, lc->dev);
 		dm_put_device(ti, lc->logdev);