kexec: dump kmessage before machine_kexec

kmsg_dump(KMSG_DUMP_SHUTDOWN) is called before machine_restart(),
machine_halt(), and 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
  ...
  [   70.914592] psci: CPU3 killed (polled 0 ms)
  [   70.915705] CPU4: shutdown
  [   70.916643] psci: CPU4 killed (polled 4 ms)
  [   70.917715] CPU5: shutdown
  [   70.918725] psci: CPU5 killed (polled 0 ms)
  [   70.919704] CPU6: shutdown
  [   70.920726] psci: CPU6 killed (polled 4 ms)
  [   70.921642] CPU7: shutdown
  [   70.922650] psci: CPU7 killed (polled 0 ms)

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Pavel Tatashin <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Reviewed-by: Petr Mladek <[email protected]>
Reviewed-by: Bhupesh Sharma <[email protected]>
Acked-by: Baoquan He <[email protected]>
Reviewed-by: Tyler Hicks <[email protected]>
Cc: James Morris <[email protected]>
Cc: Sasha Levin <[email protected]>
Cc: Eric W. Biederman <[email protected]>
Cc: Anton Vorontsov <[email protected]>
Cc: Colin Cross <[email protected]>
Cc: Tony Luck <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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