Sync reserved Illumos ioctl comment with actual number

It's 81 now.

Reviewed-by: Olaf Faaland <faaland1@llnl.gov>
Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@gmail.com>
Closes #8598

include/sys/fs/zfs.h

@@ -1192,7 +1192,7 @@ typedef enum {
  */
 typedef enum zfs_ioc {
 	/*
-	 * Illumos - 71/128 numbers reserved.
+	 * illumos - 81/128 numbers reserved.
 	 */
 	ZFS_IOC_FIRST =	('Z' << 8),
 	ZFS_IOC = ZFS_IOC_FIRST,