kernel/panic: call the 2nd crash_kexec() only if crash_kexec_post_notifiers is enabled

For compatibility with the behaviour before the commit f06e5153


("kernel/panic.c: add "crash_kexec_post_notifiers" option for kdump after
panic_notifers"), the 2nd crash_kexec() should be called only if
crash_kexec_post_notifiers is enabled.

Note that crash_kexec() returns immediately if kdump crash kernel is not
loaded, so in this case, this patch makes no functionality change, but the
point is to make it explicit, from the caller panic() side, that the 2nd
crash_kexec() does nothing.

Signed-off-by: HATAYAMA Daisuke <d.hatayama@jp.fujitsu.com>
Suggested-by: Ingo Molnar <mingo@kernel.org>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Hidehiro Kawai <hidehiro.kawai.ez@hitachi.com>
Cc: Baoquan He <bhe@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>