x86/smp: Move the call to smp_processor_id() after the early exit in native_stop_other_cpus()

Improve code generation in native_stop_other_cpus() a tiny bit:
smp_processor_id() accesses a per-CPU variable, so the compiler
is not able to move the call after the early exit on its own.

Also rename the "cpu" variable to a more descriptive "this_cpu", and
use 'cpu' as a separate iterator variable later in the function.

No functional change intended.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20231123203605.3474745-1-ubizjak@gmail.com