Fix comments on zfs_bookmark_phys

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Matt Ahrens <mahrens@delphix.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Paul Dagnelie <pcd@delphix.com>
Closes #8945

include/sys/dsl_bookmark.h

@@ -36,9 +36,11 @@ typedef struct zfs_bookmark_phys {
 	uint64_t zbm_creation_txg;	/* birth transaction group */
 	uint64_t zbm_creation_time;	/* bookmark creation time */
 
-	/* the following fields are reserved for redacted send / recv */
+	/* fields used for redacted send / recv */
 	uint64_t zbm_redaction_obj;	/* redaction list object */
 	uint64_t zbm_flags;		/* ZBM_FLAG_* */
+
+	/* fields used for bookmark written size */
 	uint64_t zbm_referenced_bytes_refd;
 	uint64_t zbm_compressed_bytes_refd;
 	uint64_t zbm_uncompressed_bytes_refd;