ARC: mm: fix leakage of memory allocated for PTE

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

ARC: mm: fix leakage of memory allocated for PTE

Since commit d9820ff ("ARC: mm: switch pgtable_t back to struct page *")

a memory leakage problem occurs. Memory allocated for page table entries

not released during process termination. This issue can be reproduced by

a small program that allocates a large amount of memory. After several

runs, you'll see that the amount of free memory has reduced and will

continue to reduce after each run. All ARC CPUs are effected by this

issue. The issue was introduced since the kernel stable release v5.15-rc1.

As described in commit d9820ff after switch pgtable_t back to struct

page *, a pointer to "struct page" and appropriate functions are used to

allocate and free a memory page for PTEs, but the pmd_pgtable macro hasn't

changed and returns the direct virtual address from the PMD (PGD) entry.

Than this address used as a parameter in the __pte_free() and as a result

this function couldn't release memory page allocated for PTEs.

Fix this issue by changing the pmd_pgtable macro and returning pointer to

struct page.