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: Dmitry 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: David S. Miller <davem@davemloft.net>