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

At the moment the top of the fixmap space is calculated from P4SEG, which
places it at the end of the store queue space when that API is enabled.
Make sure we use P3_ADDR_MAX here instead to find the proper address
limit. With this done, it's also possible to switch to the generic
vmalloc address range check now that VMALLOC_START/END encapsulate the
translatable areas that we care about.

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