util/cbmem: mmap underflow on low addresses

There is code to adjust the mapping down if a mmap fails
at a physical address. However, if the address is less
than the page size of the system then the physical offset will
underflow. This can actually cause a kernel panic on when
operating on /dev/mem.

The failing condition happens when the requested mapping at 0
fails in the kernel. The fallback path is taken and page size
is subtracted from 0 making a very large offset. The PAT code
in the kernel fails with a BUG_ON in reserve_memtype() checking
start >= end. The kernel needs to be fixed as well, but this
fallback path is wrong as well.

BUG=b:38211793

Change-Id: I32b0c15b2f1aa43fc57656d5d2d5f0e4e90e94ef
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/19679
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-by: Philippe Mathieu-Daudé <philippe.mathieu.daude@gmail.com>
Reviewed-by: Julius Werner <jwerner@chromium.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

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