sh: define TASK_UNMAPPED_BASE as a page aligned constant

b4265f12340f809447b9a48055e88c444b480c89
(mm: use vm_unmapped_area() on sh architecture)
broke sh boot. This patch define TASK_UNMAPPED_BASE
as a page aligned constant to solve this issue.
Special thanks to Michel

Acked-by: Michel Lespinasse <walken@google.com>
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>