sh: Fix up UP deadlock with SMP-aware cache ops.

This builds on top of the previous reversion and implements a special
on_each_cpu() variant that simple disables preemption across the call
while leaving the interrupt state to the function itself. There were some
unintended consequences with IRQ disabling in some of these paths on UP
that ran in to a deadlock scenario with IRQs being missed.

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