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 <[email protected]>
Acked-by: Motohiro Kosaki <[email protected]>
Acked-by: Vivek Goyal <[email protected]>
Cc: Eric Biederman <[email protected]>
Cc: Yoshihiro YUNOMAE <[email protected]>
Cc: Satoru MORIYA <[email protected]>
Cc: Tomoki Sekiyama <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>