signal/seccomp: Dump core when there is only one live thread

Replace get_nr_threads with atomic_read(&current->signal->live) as
that is a more accurate number that is decremented sooner.

Acked-by: Kees Cook <keescook@chromium.org>
Link: https://lkml.kernel.org/r/87lf6z6qbd.fsf_-_@disp2133
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>