sh: Need IRQs enabled for init_fpu().

This tosses in a local_irq_enable()/disable() pair around the init_fpu()
callsite in the FPU state restore exception handler. Fixes up a slab BUG
triggered by making a slab cache allocation that can sleep whilst
irqs_disabled(). This follows the behaviour undertaken by the x86
implementation.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>