kernel/panic.c: add "crash_kexec_post_notifiers" option for kdump after panic_notifers

kernel/panic.c: add "crash_kexec_post_notifiers" option for kdump after panic_notifers

Add a "crash_kexec_post_notifiers" boot option to run kdump after running
panic_notifiers and dump kmsg.  This can help rare situations where kdump
fails because of unstable crashed kernel or hardware failure (memory
corruption on critical data/code), or the 2nd kernel is already broken by
the 1st kernel (it's a broken behavior, but who can guarantee that the
"crashed" kernel works correctly?).

Usage: add "crash_kexec_post_notifiers" to kernel boot option.

Note that this actually increases risks of the failure of kdump.
This option should be set only if you worry about the rare case
of kdump failure rather than increasing the chance of success.

Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Acked-by: Motohiro Kosaki <Motohiro.Kosaki@us.fujitsu.com>
Acked-by: Vivek Goyal <vgoyal@redhat.com>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: Yoshihiro YUNOMAE <yoshihiro.yunomae.ez@hitachi.com>
Cc: Satoru MORIYA <satoru.moriya.br@hitachi.com>
Cc: Tomoki Sekiyama <tomoki.sekiyama@hds.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>