zfs_ioctl: avoid out-of-bound read

admbugs:	796
Submitted by:	Domagoj Stolfa <ds815@cam.ac.uk>
Reported by:	Ilja Van Sprundel <ivansprundel@ioactive.com>
Reviewed by:	avg
MFC after:	1 day

sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_ioctl.c

@@ -6440,6 +6440,10 @@ zfsdev_ioctl(struct cdev *dev, u_long zcmd, caddr_t arg, int flag,
 			cflag = ZFS_CMD_COMPAT_V28;
 			break;
 		case sizeof(zfs_cmd_v15_t):
+			if (cmd >= sizeof(zfs_ioctl_v15_to_v28) /
+			    sizeof(zfs_ioctl_v15_to_v28[0]))
+				return (EINVAL);
+
 			cflag = ZFS_CMD_COMPAT_V15;
 			vecnum = zfs_ioctl_v15_to_v28[cmd];