x86, xen: fix use of pgd_page now that it really does return a page

Impact: fix 32-bit Xen guest boot crash

On 32-bit PAE, pud_page, for no good reason, didn't really return a
struct page *.  Since Jan Beulich's fix "i386/PAE: fix pud_page()",
pud_page does return a struct page *.

Because PAE has 3 pagetable levels, the pud level is folded into the
pgd level, so pgd_page() is the same as pud_page(), and now returns
a struct page *.  Update the xen/mmu.c code which uses pgd_page()
accordingly.
Signed-off-by: NJeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

arch/x86/xen/mmu.c

@@ -874,7 +874,7 @@ static void __xen_pgd_pin(struct mm_struct *mm, pgd_t *pgd)
 #else /* CONFIG_X86_32 */
 #ifdef CONFIG_X86_PAE
 	/* Need to make sure unshared kernel PMD is pinnable */
-	xen_pin_page(mm, virt_to_page(pgd_page(pgd[pgd_index(TASK_SIZE)])),
+	xen_pin_page(mm, pgd_page(pgd[pgd_index(TASK_SIZE)]),
 		     PT_PMD);
 #endif
 	xen_do_pin(MMUEXT_PIN_L3_TABLE, PFN_DOWN(__pa(pgd)));
@@ -991,7 +991,7 @@ static void __xen_pgd_unpin(struct mm_struct *mm, pgd_t *pgd)
 
 #ifdef CONFIG_X86_PAE
 	/* Need to make sure unshared kernel PMD is unpinned */
-	xen_unpin_page(mm, virt_to_page(pgd_page(pgd[pgd_index(TASK_SIZE)])),
+	xen_unpin_page(mm, pgd_page(pgd[pgd_index(TASK_SIZE)]),
 		       PT_PMD);
 #endif