block: don't send uevent for empty disk when not invalidating

Commit 6917d068 ("block: merge invalidate_partitions into
rescan_partitions") caused a regression where systemd-udevd spins
forever using max CPU starting at boot time.

It's caused by a behavior change where a KOBJ_CHANGE uevent is now sent
in a case where previously it wasn't.

Restore the old behavior.

Fixes: 6917d068 ("block: merge invalidate_partitions into rescan_partitions")
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NEric Biggers <ebiggers@google.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

fs/block_dev.c

@@ -1531,7 +1531,7 @@ int bdev_disk_changed(struct block_device *bdev, bool invalidate)
 		ret = blk_add_partitions(disk, bdev);
 		if (ret == -EAGAIN)
 			goto rescan;
-	} else {
+	} else if (invalidate) {
 		/*
 		 * Tell userspace that the media / partition table may have
 		 * changed.