Cache colouring task_struct and kernel stack 2.5.0-0.5

Cache colouring task_struct and kernel stack 2.5.0-0.5 is really useful for it is designed to cache-colours both task_struct and kernel stack.

This patch cache-colours both task_struct and kernel stack using a slab allocator for task_struct and initial stack pointer jittering for kernel stack. An extra structure is defined for task_struct allocations as long as a new init task structure.

By using three stack color bits, eight colors will be used for initial stack frame pointer jittering that should be enough for most cache architectures. Care has to be taken in increasing too much STACK_COLOUR_BITS because this can lead to kernel stack overflows, e.g. a STACK_COLOUR_BITS set to 4 with a cache line size of 128 bytes like P4 will result in a maximum jitter of 2048 bytes by leaving only 6 Kb for the kernel stack.