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

mainline inclusion
from mainline-v5.15-rc1
commit d21918e5a94a862ccb297b9f2be38574c865fda0
category: bugfix
bugzilla: 187336, https://gitee.com/openeuler/kernel/issues/I5LCBR


CVE: NA

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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>

Conflicts:
	kernel/seccomp.c

Signed-off-by: GONG, Ruiqi <gongruiqi1@huawei.com>
Reviewed-by: Wang Weiyang <wangweiyang2@huawei.com>
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>