powerpc: add context.vdso_base for 32-bit too

This adds a vdso_base element to the mm_context_t for 32-bit compiles
(both for ARCH=powerpc and ARCH=ppc).  This fixes the compile errors
that have been reported in arch/powerpc/kernel/signal_32.c.

Signed-off-by: Paul Mackerras <paulus@samba.org>
diff --git a/include/asm-ppc/pgtable.h b/include/asm-ppc/pgtable.h
index 570b355..f886066 100644
--- a/include/asm-ppc/pgtable.h
+++ b/include/asm-ppc/pgtable.h
@@ -663,7 +663,7 @@
 	return (old & _PAGE_ACCESSED) != 0;
 }
 #define ptep_test_and_clear_young(__vma, __addr, __ptep) \
-	__ptep_test_and_clear_young((__vma)->vm_mm->context, __addr, __ptep)
+	__ptep_test_and_clear_young((__vma)->vm_mm->context.id, __addr, __ptep)
 
 #define __HAVE_ARCH_PTEP_TEST_AND_CLEAR_DIRTY
 static inline int ptep_test_and_clear_dirty(struct vm_area_struct *vma,