rationalise return value for ->hot_add_disk method.

For all array types but linear, ->hot_add_disk returns 1 on
success, 0 on failure.
For linear, it returns 0 on success and -errno on failure.

This doesn't cause a functional problem because the ->hot_add_disk
function of linear is used quite differently to the others.
However it is confusing.

So convert all to return 0 for success or -errno on failure
and fix call sites to match.

Signed-off-by: Neil Brown <neilb@suse.de>
diff --git a/drivers/md/md.c b/drivers/md/md.c
index 5d6fac1..45e255d 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -1977,10 +1977,8 @@
 			rdev->saved_raid_disk = -1;
 		err = rdev->mddev->pers->
 			hot_add_disk(rdev->mddev, rdev);
-		if (err != 1) {
+		if (err) {
 			rdev->raid_disk = -1;
-			if (err == 0)
-				return -EEXIST;
 			return err;
 		}
 		sprintf(nm, "rd%d", rdev->raid_disk);
@@ -5920,7 +5918,8 @@
 			if (rdev->raid_disk < 0
 			    && !test_bit(Faulty, &rdev->flags)) {
 				rdev->recovery_offset = 0;
-				if (mddev->pers->hot_add_disk(mddev,rdev)) {
+				if (mddev->pers->
+				    hot_add_disk(mddev, rdev) == 0) {
 					char nm[20];
 					sprintf(nm, "rd%d", rdev->raid_disk);
 					if (sysfs_create_link(&mddev->kobj,