inet_diag: bc: read cgroup id only for full sockets

Fix bug introduced by commit b1f3e43d ("inet_diag: add support for
cgroup filter").
Signed-off-by: NDmitry Yakunin <zeil@yandex-team.ru>
Reported-by: syzbot+ee80f840d9bf6893223b@syzkaller.appspotmail.com
Reported-by: syzbot+13bef047dbfffa5cd1af@syzkaller.appspotmail.com
Fixes: b1f3e43d ("inet_diag: add support for cgroup filter")
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/inet_diag.c

@@ -746,7 +746,8 @@ int inet_diag_bc_sk(const struct nlattr *bc, struct sock *sk)
 	else
 		entry.mark = 0;
 #ifdef CONFIG_SOCK_CGROUP_DATA
-	entry.cgroup_id = cgroup_id(sock_cgroup_ptr(&sk->sk_cgrp_data));
+	entry.cgroup_id = sk_fullsock(sk) ?
+		cgroup_id(sock_cgroup_ptr(&sk->sk_cgrp_data)) : 0;
 #endif
 
 	return inet_diag_bc_run(bc, &entry);