Do the irq handling in allocate_slab() instead of __slab_alloc().

__slab_alloc() is already cluttered and allocate_slab() is already
fiddling around with gfp flags.

v6->v7:
	Only increment ORDER_FALLBACK if we get a page during fallback
Signed-off-by: NChristoph Lameter <cl@linux.com>
Signed-off-by: NPekka Enberg <penberg@kernel.org>