Sequential scrub and resilver updated comments

Commit d4a72f2 which introduced multi-phase scrubs and resilvers
continued the work presented by Nexenta at the 2016 ZFS developer
summit.  Update the source to reflect their contribution.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

module/zfs/dsl_scan.c

@@ -23,6 +23,7 @@
  * Copyright (c) 2011, 2018 by Delphix. All rights reserved.
  * Copyright 2016 Gary Mills
  * Copyright (c) 2017, 2019, Datto Inc. All rights reserved.
+ * Copyright (c) 2015, Nexenta Systems, Inc. All rights reserved.
  * Copyright 2019 Joyent, Inc.
  */
 

module/zfs/metaslab.c

@@ -22,6 +22,7 @@
  * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
  * Copyright (c) 2011, 2019 by Delphix. All rights reserved.
  * Copyright (c) 2013 by Saso Kiselkov. All rights reserved.
+ * Copyright (c) 2015, Nexenta Systems, Inc. All rights reserved.
  * Copyright (c) 2017, Intel Corporation.
  */
 

module/zfs/range_tree.c

@@ -24,6 +24,7 @@
  */
 /*
  * Copyright (c) 2013, 2019 by Delphix. All rights reserved.
+ * Copyright (c) 2015, Nexenta Systems, Inc. All rights reserved.
  */
 
 #include <sys/zfs_context.h>