x86, mm: do not leak page->ptl for pmd page tables

x86, mm: do not leak page->ptl for pmd page tables

There are two code paths how page with pmd page table can be freed:
pmd_free() and pmd_free_tlb().

I've missed the second one and didn't add page table destructor call
there.  It leads to leak of page->ptl for pmd page tables, if
dynamically allocated page->ptl is in use.

The patch adds the missed destructor and modifies documentation
accordingly.

Signed-off-by: Kirill A. Shutemov <[email protected]>
Reported-by: Andrey Vagin <[email protected]>
Tested-by: Andrey Vagin <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>