slqb enables interrupts very early

[Sebastian Andrzej Siewior <sebastian@breakpoint.cc>]

I've checkout slab-2.6/linux-next and noticed that the interrupts are
enabled very early by accident. Please look at the following call stack:

    start_kernel()
    kmem_cache_init()
    kmem_cache_open()
    down_write(&slqb_lock);
    __down_write()
    __down_write_nested()

Now, __down_write_nested() protects its internal structure the follwing
way:
    spin_lock_irq(&sem->wait_lock);
    ...
    spin_unlock_irq(&sem->wait_lock);

so once we return, we return with interrupts on.

Sebastian