ARM: 7641/1: memory: fix broken mmap by ensuring TASK_UNMAPPED_BASE is aligned

We have received multiple reports of mmap failures when running with a
2:2 vm split. These manifest as either -EINVAL with a non page-aligned
address (ending 0xaaa) or a SEGV, depending on the application. The
issue is commonly observed in children of make, which appears to use
bottom-up mmap (assumedly because it changes the stack rlimit).

Further investigation reveals that this regression was triggered by
394ef640 ("mm: use vm_unmapped_area() on arm architecture"), whereby
TASK_UNMAPPED_BASE is no longer page-aligned for bottom-up mmap, causing
get_unmapped_area to choke on misaligned addressed.

This patch fixes the problem by defining TASK_UNMAPPED_BASE in terms of
TASK_SIZE and explicitly aligns the result to 16M, matching the other
end of the heap.

Acked-by: Nicolas Pitre <nico@linaro.org>
Reported-by: Steve Capper <steve.capper@arm.com>
Reported-by: Jean-Francois Moine <moinejf@free.fr>
Reported-by: Christoffer Dall <cdall@cs.columbia.edu>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>