um: Call pgtable_pmd_page_dtor() in __pmd_free_tlb()

Commit b2b29d6d

 ("mm: account PMD tables like PTE tables") uncovered
a bug in uml, we forgot to call the destructor.
While we are here, give x a sane name.

Reported-by: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Co-developed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Richard Weinberger <richard@nod.at>
Tested-by: Christopher Obbard <chris.obbard@collabora.com>