From: Manfred Spraul <manfred@colorfullife.com>

kernel/fork.c contains a disabled cache for task stuctures.  task
structures are placed into the task cache only if "tsk==current", and
"tsk==current" is impossible.  There is even a WARN_ON against that in
__put_task_struct().

So remove it entirely - it's dead code.

One problem is that order-1 allocations are not cached per-cpu - we can
use kmalloc for the stack.