linux/acpi: adjust extcntl's changes to (mostly) common code, mostly for readability

linux/acpi: adjust extcntl's changes to (mostly) common code, mostly for readability

While I realize that it may have been upon my request that all these
#ifdef-s were introduced, I think this went too far here: With the
abstraction in include/acpi/processor.h in place, the code still
guarantees even without all these #ifdef-s that in native kernels
built from the same source the logic will not change.

The one thing that appears a little weak still is the NR_ACPI_CPUS
construct - nothing guarantees that the ACPI IDs are constrained to 8
bits, and hence using an array here doesn't seem either safe or
scalable looking forward. To make the issue explict, the patch adds a
respective BUG_ON().

Also remove a stray export.

Signed-off-by: Jan Beulich <jbeulich@novell.com>