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_-_@disp2133Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

kernel/seccomp.c

@@ -1264,7 +1264,7 @@ static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd,
 		seccomp_log(this_syscall, SIGSYS, action, true);
 		/* Dump core only if this is the last remaining thread. */
 		if (action != SECCOMP_RET_KILL_THREAD ||
-		    get_nr_threads(current) == 1) {
+		    (atomic_read(&current->signal->live) == 1)) {
 			/* Show the original registers in the dump. */
 			syscall_rollback(current, current_pt_regs());
 			/* Trigger a coredump with SIGSYS */