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/arch/powerpc/mm/ppc_mmu_32.c b/arch/powerpc/mm/ppc_mmu_32.c
index ed7fcfe..1df731e 100644
--- a/arch/powerpc/mm/ppc_mmu_32.c
+++ b/arch/powerpc/mm/ppc_mmu_32.c
@@ -190,7 +190,7 @@
 		return;
 	pmd = pmd_offset(pgd_offset(mm, ea), ea);
 	if (!pmd_none(*pmd))
-		add_hash_page(mm->context, ea, pmd_val(*pmd));
+		add_hash_page(mm->context.id, ea, pmd_val(*pmd));
 }
 
 /*