x86: Use memblock_set_current_limit() to set limit for memblock.

In setup_arch() of x86, it set memblock.current_limit directly.
We should use memblock_set_current_limit(). If the implementation
is changed, it is easy to maintain.

Signed-off-by: Tang Chen <tangchen@cn.fujitsu.com>
Link: http://lkml.kernel.org/r/1376451844-15682-1-git-send-email-tangchen@cn.fujitsu.com
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>