md/md-linear: Fix a NULL vs IS_ERR() bug in linear_add()

The linear_conf() returns error pointers, it doesn't return NULL.  Update
the error checking to match.

Fixes: 127186c ("md: reintroduce md-linear")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/add654be-759f-4b2d-93ba-a3726dae380c@stanley.mountain
Signed-off-by: Song Liu <song@kernel.org>

Author: Dan Carpenter

drivers/md/md-linear.c

@@ -204,8 +204,8 @@ static int linear_add(struct mddev *mddev, struct md_rdev *rdev)
 	rdev->saved_raid_disk = -1;
 
 	newconf = linear_conf(mddev, mddev->raid_disks + 1);
-	if (!newconf)
-		return -ENOMEM;
+	if (IS_ERR(newconf))
+		return PTR_ERR(newconf);
 
 	/* newconf->raid_disks already keeps a copy of * the increased
 	 * value of mddev->raid_disks, WARN_ONCE() is just used to make