LoongArch: mm: Refactor TLB exception handlers

This patch simplifies TLB load, store and modify exception handlers:

1. Reduce instructions, such as alu/csr and memory access;
2. Execute tlb search instruction only in the fast path;
3. Return directly from the fast path for both normal and huge pages;
4. Re-tab the assembly for better vertical alignment.

And fixes the concurrent modification issue of fast path for huge pages.

This issue will occur in the following steps:

   CPU-1 (In TLB exception)         CPU-2 (In THP splitting)
1: Load PMD entry (HUGE=1)
2: Goto huge path
3:                                  Store PMD entry (HUGE=0)
4: Reload PMD entry (HUGE=0)
5: Fill TLB entry (PA is incorrect)

This patch also slightly improves the TLB processing performance:

* Normal pages: 2.15%, Huge pages: 1.70%.

  #include <stdio.h>
  #include <stdlib.h>
  #include <unistd.h>
  #include <sys/mman.h>

  int main(int argc, char *argv[])
  {
        size_t page_size;
        size_t mem_size;
        size_t off;
        void *base;
        int flags;
        int i;

        if (argc < 2) {
                fprintf(stderr, "%s MEM_SIZE [HUGE]\n", argv[0]);
                return -1;
        }

        page_size = sysconf(_SC_PAGESIZE);
        flags = MAP_PRIVATE | MAP_ANONYMOUS;
        mem_size = strtoul(argv[1], NULL, 10);
        if (argc > 2)
                flags |= MAP_HUGETLB;

        for (i = 0; i < 10; i++) {
                base = mmap(NULL, mem_size, PROT_READ, flags, -1, 0);
                if (base == MAP_FAILED) {
                        fprintf(stderr, "Map memory failed!\n");
                        return -1;
                }

                for (off = 0; off < mem_size; off += page_size)
                        *(volatile int *)(base + off);

                munmap(base, mem_size);
        }

        return 0;
  }

Signed-off-by: Rui Wang <wangrui@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>