Revert "sh: Ensure fixmap and store queue space can co-exist."

This reverts commit 20e7c297.
With store queues enabled the area above P4SEG has special properties
from the MMU's point of view, which was causing fixmap failure. We'll
have to do something else to satisfy the vmalloc range check.

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