block: update some copyrights

Update copyrights for files that have gotten some major rewrites lately.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NJohannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

block/genhd.c

 // SPDX-License-Identifier: GPL-2.0
 /*
  *  gendisk handling
+ *
+ * Portions Copyright (C) 2020 Christoph Hellwig
  */
 
 #include <linux/module.h>

block/partitions/core.c

@@ -2,6 +2,7 @@
 /*
  * Copyright (C) 1991-1998  Linus Torvalds
  * Re-organised Feb 1998 Russell King
+ * Copyright (C) 2020 Christoph Hellwig
  */
 #include <linux/fs.h>
 #include <linux/slab.h>

fs/block_dev.c

@@ -2,6 +2,7 @@
 /*
  *  Copyright (C) 1991, 1992  Linus Torvalds
  *  Copyright (C) 2001  Andrea Arcangeli <andrea@suse.de> SuSE
+ *  Copyright (C) 2016 - 2020 Christoph Hellwig
  */
 
 #include <linux/init.h>