Revert "sh: Kill off now redundant local irq disabling."

This reverts commit 64a6d722.

Unfortunately we can't use on_each_cpu() for all of the cache ops, as
some of them only require preempt disabling. This seems to be the same
issue that impacts the mips r4k caches, where this code was based on.
This fixes up a deadlock that showed up in some IRQ context cases.

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