LoongArch: Fix build error due to backport

LoongArch: Fix build error due to backport

In 6.1 there is no pmdp_get() definition, so use *pmd directly, in order
to avoid such build error due to a recently backport:

arch/loongarch/mm/hugetlbpage.c: In function 'huge_pte_offset':
arch/loongarch/mm/hugetlbpage.c:50:25: error: implicit declaration of function 'pmdp_get'; did you mean 'ptep_get'?  [-Wimplicit-function-declaration]
   50 |         return pmd_none(pmdp_get(pmd)) ? NULL : (pte_t *) pmd;
      |                         ^~~~~~~~
      |                         ptep_get

Reported-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/f978ec9a-b103-40af-b116-6a9238197110@roeck-us.net
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>