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 965278d ("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: 965278d ("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>

arch/arm/mm/mmu.c

@@ -1218,15 +1218,15 @@ void __init adjust_lowmem_bounds(void)
 
 	high_memory = __va(arm_lowmem_limit - 1) + 1;
 
+	if (!memblock_limit)
+		memblock_limit = arm_lowmem_limit;
+
 	/*
 	 * Round the memblock limit down to a pmd size.  This
 	 * helps to ensure that we will allocate memory from the
 	 * last full pmd, which should be mapped.
 	 */
-	if (memblock_limit)
-		memblock_limit = round_down(memblock_limit, PMD_SIZE);
-	if (!memblock_limit)
-		memblock_limit = arm_lowmem_limit;
+	memblock_limit = round_down(memblock_limit, PMD_SIZE);
 
 	if (!IS_ENABLED(CONFIG_HIGHMEM) || cache_is_vipt_aliasing()) {
 		if (memblock_end_of_DRAM() > arm_lowmem_limit) {