x86, memblock: Add memblock_x86_free_memory_in_range()

It will return free memory size in specified range.

We can not use memory_size - reserved_size here, because some reserved area
may not be in the scope of memblock.memory.region.

Use memblock.memory.region subtracting memblock.reserved.region to get free range array.
then count size of all free ranges.

-v2: Ben insist on using _in_range

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>