LoongArch: Use __pmd()/__pte() for swap entry conversions

LoongArch: Use __pmd()/__pte() for swap entry conversions

The __pmd() and __pte() helper macros provide the correct initialization
syntax and abstraction for the pmd_t and pte_t types.

Use __pmd() to fix follow warning about __swp_entry_to_pmd() with gcc-15
under specific configs [1] :

  In file included from ./include/linux/pgtable.h:6,
                   from ./include/linux/mm.h:31,
                   from ./include/linux/pagemap.h:8,
                   from arch/loongarch/mm/init.c:14:
  ./include/linux/swapops.h: In function ‘swp_entry_to_pmd’:
  ./arch/loongarch/include/asm/pgtable.h:302:34: error: missing braces around initializer [-Werror=missing-braces]
    302 | #define __swp_entry_to_pmd(x)   ((pmd_t) { (x).val | _PAGE_HUGE })
        |                                  ^
  ./include/linux/swapops.h:559:16: note: in expansion of macro ‘__swp_entry_to_pmd’
    559 |         return __swp_entry_to_pmd(arch_entry);
        |                ^~~~~~~~~~~~~~~~~~
  cc1: all warnings being treated as errors

Also update __swp_entry_to_pte() to use __pte() for consistency.

[1]. https://download.01.org/0day-ci/archive/20251119/202511190316.luI90kAo-lkp@intel.com/config

Cc: stable@vger.kernel.org
Signed-off-by: Yuli Wang <wangyl5933@chinaunicom.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>