reboot: refactor and comment the cpu selection code

Small improvements to the code, without changing the way it works:

 - use a local variable, to avoid a small time lapse where reboot_cpu
   can have an invalid value

 - comment the code which is not easy to understand at a glance

 - merge two identical code blocks into one

 - replace pointer arithmetics with equivalent array syntax

Link: https://lkml.kernel.org/r/20201103214025.116799-4-mcroce@linux.microsoft.com


Signed-off-by: Matteo Croce <mcroce@microsoft.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Fabian Frederick <fabf@skynet.be>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: Kees Cook <keescook@chromium.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Pavel Tatashin <pasha.tatashin@soleen.com>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Robin Holt <robinmholt@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>