Revert "mm: create a separate slab for page->ptl allocation"

This reverts commit ea1e7ed33708c7a760419ff9ded0a6cb90586a50.

Al points out that while the commit *does* actually create a separate
slab for the page->ptl allocation, that slab is never actually used, and
the code continues to use kmalloc/kfree.

Damien Wyart points out that the original patch did have the conversion
to use kmem_cache_alloc/free, so it got lost somewhere on its way to me.

Revert the half-arsed attempt that didn't do anything.  If we really do
want the special slab (remember: this is all relevant just for debug
builds, so it's not necessarily all that critical) we might as well redo
the patch fully.

Reported-by: Al Viro <viro@zeniv.linux.org.uk>
Acked-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Kirill A Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/init/main.c b/init/main.c
index 01573fd..febc511 100644
--- a/init/main.c
+++ b/init/main.c
@@ -476,7 +476,7 @@
 	mem_init();
 	kmem_cache_init();
 	percpu_init_late();
-	pgtable_init();
+	pgtable_cache_init();
 	vmalloc_init();
 }