powerpc/mm: Move around mmu_gathers definition on 64-bit

The definition for the global structure mmu_gathers, used by generic code,
is currently defined in multiple places not including anything used by
64-bit Book3E. This changes it by moving to one place common to all
processors.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
diff --git a/arch/powerpc/mm/pgtable.c b/arch/powerpc/mm/pgtable.c
index a65979a..cafb2a2 100644
--- a/arch/powerpc/mm/pgtable.c
+++ b/arch/powerpc/mm/pgtable.c
@@ -30,6 +30,8 @@
 #include <asm/tlbflush.h>
 #include <asm/tlb.h>
 
+DEFINE_PER_CPU(struct mmu_gather, mmu_gathers);
+
 #ifdef CONFIG_SMP
 
 /*