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>
        struct hlist_head *head;
        struct sock *osk;
        struct hlist_node *node;
-       s32 pid = current->tgid;
+       s32 pid = task_tgid_vnr(current);
        int err;
        static s32 rover = -4097;