kexec: dump kmessage before machine_kexec

kexec: dump kmessage before machine_kexec

kmsg_dump(KMSG_DUMP_SHUTDOWN) is called before
machine_restart(), machine_halt(), machine_power_off(), the only one that
is missing is  machine_kexec().

The dmesg output that it contains can be used to study the shutdown
performance of both kernel and systemd during kexec reboot.

Here is example of dmesg data collected after kexec:

root@dplat-cp22:~# cat /sys/fs/pstore/dmesg-ramoops-0 | tail
...
<6>[   70.914592] psci: CPU3 killed (polled 0 ms)
<5>[   70.915705] CPU4: shutdown
<6>[   70.916643] psci: CPU4 killed (polled 4 ms)
<5>[   70.917715] CPU5: shutdown
<6>[   70.918725] psci: CPU5 killed (polled 0 ms)
<5>[   70.919704] CPU6: shutdown
<6>[   70.920726] psci: CPU6 killed (polled 4 ms)
<5>[   70.921642] CPU7: shutdown
<6>[   70.922650] psci: CPU7 killed (polled 0 ms)

Link: https://lkml.kernel.org/r/20210319192326.146000-2-pasha.tatashin@soleen.com
Signed-off-by: Pavel Tatashin <pasha.tatashin@soleen.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Reviewed-by: Bhupesh Sharma <bhsharma@redhat.com>
Acked-by: Baoquan He <bhe@redhat.com>
Reviewed-by: Tyler Hicks <tyhicks@linux.microsoft.com>
Cc: James Morris <jmorris@namei.org>
Cc: Sasha Levin <sashal@kernel.org>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Anton Vorontsov <anton@enomsg.org>
Cc: Colin Cross <ccross@android.com>
Cc: Tony Luck <tony.luck@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>