Honor the 'force' flag for set_affinity, by selecting a CPU
from the given mask (which may not be reported "online" by
the cpu_online_mask). Some drivers, like ARM PMU, rely on it.

Cc: Marc Zyngier <marc.zyngier@arm.com>
Reported-by: NMark Rutland <mark.rutland@arm.com>
Signed-off-by: NSuzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: NMarc Zyngier <marc.zyngier@arm.com>