timer.c: Optimise apply_slack() for size and speed.

http://lkml.indianaedu/hypermail/linux/kernel/1201.3/03172.html

To apply proper slack, the original algorithm used to prepare a mask and
then apply the mask to obtain the appropriately rounded-off absolute
time the timer expires.

This patch modifies the masking logic to a bit-shift logic, therby
reducing the complexity and number of operations. Thus obtaining a minor
speed-up.
irq: Set CPU affinity as soon as the irq thread is created
In an embedded target where IRQ affinities are pre-configured during boot-time (and not modified through /proc/irq/../smp_affinity) during run-time, the extra latency due to irq_thread_check_affinity can be avoided during processing of the first interrupt with this patch.

backported from Linux 3.5+ by faux123