LoongArch: Fix panic caused by NULL-PMD in huge_pte_offset()

In the Linux kernel, the following vulnerability has been resolved:

LoongArch: Fix panic caused by NULL-PMD in huge_pte_offset()

ERROR INFO:

CPU 25 Unable to handle kernel paging request at virtual address 0x0

...

Call Trace:

[<900000000023c30c>] huge_pte_offset+0x3c/0x58

[<900000000057fd4c>] hugetlb_follow_page_mask+0x74/0x438

[<900000000051fee8>] __get_user_pages+0xe0/0x4c8

[<9000000000522414>] faultin_page_range+0x84/0x380

[<9000000000564e8c>] madvise_vma_behavior+0x534/0xa48

[<900000000056689c>] do_madvise+0x1bc/0x3e8

[<9000000000566df4>] sys_madvise+0x24/0x38

[<90000000015b9e88>] do_syscall+0x78/0x98

[<9000000000221f18>] handle_syscall+0xb8/0x158

In some cases, pmd may be NULL and rely on NULL as the return value for

processing, so it is necessary to determine this situation here.