ARM: 7785/1: mm: restrict early_alloc to section-aligned memory

When map_lowmem() runs, and processes a memory bank whose start or end
is not section-aligned, memory must be allocated to store the 2nd-level
page tables. Those allocations are made by calling memblock_alloc().

At this point, the only memory that is free *and* mapped is memory which
has already been mapped by map_lowmem() itself. For this reason, we must
calculate the first point at which map_lowmem() will need to allocate
memory, and set the memblock allocation limit to a lower address, so that
memblock_alloc() is guaranteed to return memory that is already mapped.

This patch enhances sanity_check_meminfo() to calculate that memory
address, and pass it to memblock_set_current_limit(), rather than just
assuming the limit is arm_lowmem_limit.

The algorithm applied is:

* Default memblock_limit to arm_lowmem_limit in the absence of any other
  limit; arm_lowmem_limit is the highest memory that is mapped by
  map_lowmem().

* While walking the list of memblocks, if the start of a block is not
  aligned, 2nd-level page tables will need to be allocated to map the
  first few pages of the block. Hence, the memblock_limit must be before
  the start of the block.

* Similarly, if the end of any block is not aligned, 2nd-level page
  tables will need to be allocated to map the last few pages of the
  block. Hence, the memblock_limit must point at the end of the block,
  rounded down to section-alignment.

* The memory blocks are assumed to be sorted in address order, so the
  first unaligned block start or end is used to set the limit.

With this algorithm, the start or end of almost any bank can be non-
section-aligned. The only exception is that the start of bank 0 must
be section-aligned, since otherwise memory would need to be allocated
when mapping the start of bank 0, which occurs before any free memory
is mapped.

[swarren, wrote commit description, rewrote calculation of memblock_limit]

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>