unix: add cgroupstats type and constants

The cgroupstats structure is available via Linux's generic netlink
taskstats interface.

For more information, please see:
https://www.kernel.org/doc/Documentation/accounting/cgroupstats.txt.

Fixes golang/go#23388.

Change-Id: I8a05a3875209e99a427d54049eacc98f415e44cf
Reviewed-on: https://go-review.googlesource.com/86975
Run-TryBot: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

unix/mkerrors.sh

@@ -426,6 +426,7 @@ ccflags="$@"
 		$2 ~ /^(VM|VMADDR)_/ ||
 		$2 ~ /^IOCTL_VM_SOCKETS_/ ||
 		$2 ~ /^(TASKSTATS|TS)_/ ||
+		$2 ~ /^CGROUPSTATS_/ ||
 		$2 ~ /^GENL_/ ||
 		$2 ~ /^UTIME_/ ||
 		$2 ~ /^XATTR_(CREATE|REPLACE)/ ||