This was included in OpenVZ kernels but wasn't integrated upstream.
>From git://git.openvz.org/pub/linux-2.6.24-openvz:

	commit 5c69402f18adf7276352e051ece2cf31feefab02
	Author: Alexey Dobriyan <adobriyan@openvz.org>
	Date:   Mon Dec 24 14:37:45 2007 +0300

	    netlink: fixup ->tgid to work in multiple PID namespaces
Signed-off-by: Tom Goff <thomas.goff@boeing.com>
Acked-by: Alexey Dobriyan <adobriyan@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>