LoongArch: Return NULL from huge_pte_offset() for invalid PMD

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

LoongArch: Return NULL from huge_pte_offset() for invalid PMD

LoongArch's huge_pte_offset() currently returns a pointer to a PMD slot

even if the underlying entry points to invalid_pte_table (indicating no

mapping). Callers like smaps_hugetlb_range() fetch this invalid entry

value (the address of invalid_pte_table) via this pointer.

The generic is_swap_pte() check then incorrectly identifies this address

as a swap entry on LoongArch, because it satisfies the "!pte_present()

&& !pte_none()" conditions. This misinterpretation, combined with a

coincidental match by is_migration_entry() on the address bits, leads to

kernel crashes in pfn_swap_entry_to_page().

Fix this at the architecture level by modifying huge_pte_offset() to

check the PMD entry's content using pmd_none() before returning. If the

entry is invalid (i.e., it points to invalid_pte_table), return NULL

instead of the pointer to the slot.