slub: Avoid disabling interrupts in free slowpath

Disabling interrupts can be avoided now. However, list operation still require
disabling interrupts since allocations can occur from interrupt
contexts and there is no way to perform atomic list operations.

The acquition of the list_lock therefore has to disable interrupts as well.

Dropping interrupt handling significantly simplifies the slowpath.

Signed-off-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>