ARM: 8685/1: ensure memblock-limit is pmd-aligned

The pmd containing memblock_limit is cleared by prepare_page_table()
which creates the opportunity for early_alloc() to allocate unmapped
memory if memblock_limit is not pmd aligned causing a boot-time hang.

Commit 965278dc ("ARM: 8356/1: mm: handle non-pmd-aligned end of RAM")
attempted to resolve this problem, but there is a path through the
adjust_lowmem_bounds() routine where if all memory regions start and
end on pmd-aligned addresses the memblock_limit will be set to
arm_lowmem_limit.

Since arm_lowmem_limit can be affected by the vmalloc early parameter,
the value of arm_lowmem_limit may not be pmd-aligned. This commit
corrects this oversight such that memblock_limit is always rounded
down to pmd-alignment.

Fixes: 965278dc ("ARM: 8356/1: mm: handle non-pmd-aligned end of RAM")
Signed-off-by: Doug Berger <opendmb@gmail.com>
Suggested-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>