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>

arch/um/include/asm/pgalloc.h

@@ -33,7 +33,13 @@ do {							\
 } while (0)
 
 #ifdef CONFIG_3_LEVEL_PGTABLES
-#define __pmd_free_tlb(tlb,x, address)   tlb_remove_page((tlb),virt_to_page(x))
+
+#define __pmd_free_tlb(tlb, pmd, address)		\
+do {							\
+	pgtable_pmd_page_dtor(virt_to_page(pmd));	\
+	tlb_remove_page((tlb),virt_to_page(pmd));	\
+} while (0)						\
+
 #endif
 
 #endif