ARM: 8356/1: mm: handle non-pmd-aligned end of RAM

At boot time we round the memblock limit down to section size in an
attempt to ensure that we will have mapped this RAM with section
mappings prior to allocating from it. When mapping RAM we iterate over
PMD-sized chunks, creating these section mappings.

Section mappings are only created when the end of a chunk is aligned to
section size. Unfortunately, with classic page tables (where PMD_SIZE is
2 * SECTION_SIZE) this means that if a chunk is between 1M and 2M in
size the first 1M will not be mapped despite having been accounted for
in the memblock limit. This has been observed to result in page tables
being allocated from unmapped memory, causing boot-time hangs.

This patch modifies the memblock limit rounding to always round down to
PMD_SIZE instead of SECTION_SIZE. For classic MMU this means that we
will round the memblock limit down to a 2M boundary, matching the limits
on section mappings, and preventing allocations from unmapped memory.
For LPAE there should be no change as PMD_SIZE == SECTION_SIZE.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reported-by: Stefan Agner <stefan@agner.ch>
Tested-by: Stefan Agner <stefan@agner.ch>
Acked-by: Laura Abbott <labbott@redhat.com>
Tested-by: Hans de Goede <hdegoede@redhat.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Steve Capper <steve.capper@linaro.org>
Cc: stable@vger.kernel.org
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>