Enhance comments for large dnode project

Fix a few nits in the comments from large dnodes. Also import
some of the commit message as a comment in the code, making
it more accessible.

Reviewed-by: @rottegift 
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Wilson <george.wilson@delphix.com>
Signed-off-by: Matt Ahrens <mahrens@delphix.com>
Closes #6551

module/zfs/dmu_object.c

@@ -318,7 +318,7 @@ dmu_object_next(objset_t *os, uint64_t *objectp, boolean_t hole, uint64_t txg)
 			dmu_object_info_t doi;
 
 			error = dmu_object_info(os, i, &doi);
-			if (error)
+			if (error != 0)
 				skip = 1;
 			else
 				skip = doi.doi_dnodesize >> DNODE_SHIFT;

module/zfs/dnode.c

@@ -1176,6 +1176,18 @@ dnode_rele_slots(dnode_children_t *children, int idx, int slots)
 }
 
 /*
+ * When the DNODE_MUST_BE_FREE flag is set, the "slots" parameter is used
+ * to ensure the hole at the specified object offset is large enough to
+ * hold the dnode being created. The slots parameter is also used to ensure
+ * a dnode does not span multiple dnode blocks. In both of these cases, if
+ * a failure occurs, ENOSPC is returned. Keep in mind, these failure cases
+ * are only possible when using DNODE_MUST_BE_FREE.
+ *
+ * If the DNODE_MUST_BE_ALLOCATED flag is set, "slots" must be 0.
+ * dnode_hold_impl() will check if the requested dnode is already consumed
+ * as an extra dnode slot by an large dnode, in which case it returns
+ * ENOENT.
+ *
  * errors:
  * EINVAL - invalid object number.
  * ENOSPC - hole too small to fulfill "slots" request

module/zfs/zfs_replay.c

@@ -21,7 +21,7 @@
 /*
  * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
  * Copyright (c) 2012 Cyril Plisko. All rights reserved.
- * Copyright (c) 2013, 2015 by Delphix. All rights reserved.
+ * Copyright (c) 2013, 2017 by Delphix. All rights reserved.
  */
 
 #include <sys/types.h>
@@ -453,8 +453,8 @@ zfs_replay_create(zfsvfs_t *zfsvfs, lr_create_t *lr, boolean_t byteswap)
 	 * eventually end up in zfs_mknode(), which assigns the object's
 	 * creation time, generation number, and dnode slot count. The
 	 * generic zfs_create() has no concept of these attributes, so
-	 * we smuggle the values inside * the vattr's otherwise unused
-	 * va_ctime, va_nblocks, and va_nlink fields.
+	 * we smuggle the values inside the vattr's otherwise unused
+	 * va_ctime, va_nblocks, and va_fsid fields.
 	 */
 	ZFS_TIME_DECODE(&xva.xva_vattr.va_ctime, lr->lr_crtime);
 	xva.xva_vattr.va_nblocks = lr->lr_gen;