memblock: Use memblock_reserve() in memblock internal functions

Make memblock_double_array(), __memblock_alloc_base() and
memblock_alloc_nid() use memblock_reserve() instead of calling
memblock_add_region() with reserved array directly.  This eases
debugging and updates to memblock_add_region().

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Yinghai Lu <yinghai@kernel.org>