powerpc/mm: Rework & cleanup page table freeing code path

That patch used to just add a hook to page table flushing but
pulling that string brought out a whole bunch of issues, so it
now does that and more:

 - We now make the RCU batching of page freeing SMP only, as I
believe it was intended initially. We make a few more things compile
to nothing on !CONFIG_SMP

 - Some macros are turned into functions, though that forced me to
out of line a few stuffs due to unsolvable include depenencies,
however it's probably better that way anyway, it's not -that-
critical code path.

 - 32-bit didn't call pte_free_finish() on tlb_flush() which means
that it wouldn't push out the batch to RCU for delayed freeing when
a bunch of page tables have been freed, they would just stay in there
until the batch gets full.

64-bit BookE will use that hook to maintain the virtually linear
page tables or the indirect entries in the TLB when using the
HW loader.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/mm/pgtable.c

@@ -30,6 +30,14 @@
 #include <asm/tlbflush.h>
 #include <asm/tlb.h>
 
+#ifdef CONFIG_SMP
+
+/*
+ * Handle batching of page table freeing on SMP. Page tables are
+ * queued up and send to be freed later by RCU in order to avoid
+ * freeing a page table page that is being walked without locks
+ */
+
 static DEFINE_PER_CPU(struct pte_freelist_batch *, pte_freelist_cur);
 static unsigned long pte_freelist_forced_free;
 
@@ -116,6 +124,8 @@ void pte_free_finish(void)
 	*batchp = NULL;
 }
 
+#endif /* CONFIG_SMP */
+
 /*
  * Handle i/d cache flushing, called from set_pte_at() or ptep_set_access_flags()
  */