core-parking: interact with runtime SMT-disabling

core-parking: interact with runtime SMT-disabling

When disabling SMT at runtime, secondary threads should no longer be
candidates for bringing back up in response to _PUR ACPI events. Purge
them from the tracking array.

Doing so involves adding locking to guard accounting data in the core
parking code. While adding the declaration for the lock, take the
liberty to drop two unnecessary forward function declarations.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>