mm/vmalloc: query dynamic DEBUG_PAGEALLOC setting

As CONFIG_DEBUG_PAGEALLOC can be enabled/disabled via kernel parameters
we can optimize some cases by checking the enablement state.

This is follow-up work for Christian's Optimize CONFIG_DEBUG_PAGEALLOC:

  https://lkml.org/lkml/2016/1/27/194

Remaining work is to make sparc to be aware of this but it looks not
easy for me so I skip that in this series.

This patch (of 5):

We can disable debug_pagealloc processing even if the code is complied
with CONFIG_DEBUG_PAGEALLOC.  This patch changes the code to query
whether it is enabled or not in runtime.

[akpm@linux-foundation.org: update comment, per David.  Adjust comment to use 80 cols]
Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Acked-by: David Rientjes <rientjes@google.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Takashi Iwai <tiwai@suse.com>
Cc: Chris Metcalf <cmetcalf@ezchip.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/vmalloc.c

@@ -531,22 +531,21 @@ static void unmap_vmap_area(struct vmap_area *va)
 static void vmap_debug_free_range(unsigned long start, unsigned long end)
 {
 	/*
-	 * Unmap page tables and force a TLB flush immediately if
-	 * CONFIG_DEBUG_PAGEALLOC is set. This catches use after free
-	 * bugs similarly to those in linear kernel virtual address
-	 * space after a page has been freed.
+	 * Unmap page tables and force a TLB flush immediately if pagealloc
+	 * debugging is enabled.  This catches use after free bugs similarly to
+	 * those in linear kernel virtual address space after a page has been
+	 * freed.
 	 *
-	 * All the lazy freeing logic is still retained, in order to
-	 * minimise intrusiveness of this debugging feature.
+	 * All the lazy freeing logic is still retained, in order to minimise
+	 * intrusiveness of this debugging feature.
 	 *
-	 * This is going to be *slow* (linear kernel virtual address
-	 * debugging doesn't do a broadcast TLB flush so it is a lot
-	 * faster).
+	 * This is going to be *slow* (linear kernel virtual address debugging
+	 * doesn't do a broadcast TLB flush so it is a lot faster).
 	 */
-#ifdef CONFIG_DEBUG_PAGEALLOC
+	if (debug_pagealloc_enabled()) {
 		vunmap_page_range(start, end);
 		flush_tlb_kernel_range(start, end);
-#endif
+	}
 }
 
 /*