LoongArch: numa: Fix high_memory calculation

For 64bit kernel without HIGHMEM, high_memory is the virtual address of
the highest physical address in the system. But __va(get_num_physpages()
<< PAGE_SHIFT) is not what we want for high_memory because there may be
holes in the physical address space. On the other hand, max_low_pfn is
calculated from memblock_end_of_DRAM(), which is exactly corresponding
to the highest physical address, so use it for high_memory calculation.

Cc: <stable@vger.kernel.org>
Fixes: d4b6f156

 ("LoongArch: Add Non-Uniform Memory Access (NUMA) support")
Signed-off-by: Chong Qiao <qiaochong@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>