LoongArch: Prefer top-down allocation after arch_mem_init()

Currently we use bottom-up allocation after sparse_init(), the reason is
sparse_init() need a lot of memory, and bottom-up allocation may exhaust
precious low memory (below 4GB). On the other hand, SWIOTLB and CMA need
low memories for DMA32, so swiotlb_init() and dma_contiguous_reserve()
need bottom-up allocation.

Since swiotlb_init() and dma_contiguous_reserve() are both called in
arch_mem_init(), we no longer need bottom-up allocation after that. So
we set the allocation policy to top-down at the end of arch_mem_init(),
in order to avoid later memory allocations (such as KASAN) exhaust low
memory.

This solve at least two problems:
1. Some buggy BIOSes use 0xfd000000~0xfe000000 for secondary CPUs, but
didn't reserve this range, which causes smpboot failures.
2. Some DMA32 devices, such as Loongson-DRM and OHCI, cannot work with
KASAN enabled.

Cc: stable@vger.kernel.org
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>