linux(4): Handle FUTEX_LOCK_PI2 oeration.

FUTEX_LOCK_PI2 was added to support clock selection as FUTEX_LOCK_PI uses a
CLOCK_REALTIME based absolute value since it was implemented, but it does not
require that the FUTEX_CLOCK_REALTIME bit is set, because that was introduced
later.

MFC after: 2 weeks