page allocator: do not sanity check order in the fast path

No user of the allocator API should be passing in an order >= MAX_ORDER
but we check for it on each and every allocation.  Delete this check and
make it a VM_BUG_ON check further down the call path.

[akpm@linux-foundation.org: s/VM_BUG_ON/WARN_ON_ONCE/]
Signed-off-by: Mel Gorman <mel@csn.ul.ie>
Reviewed-by: Christoph Lameter <cl@linux-foundation.org>
Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Reviewed-by: Pekka Enberg <penberg@cs.helsinki.fi>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: Dave Hansen <dave@linux.vnet.ibm.com>
Cc: Lee Schermerhorn <Lee.Schermerhorn@hp.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_alloc.c

@@ -1401,6 +1401,9 @@ get_page_from_freelist(gfp_t gfp_mask, nodemask_t *nodemask, unsigned int order,
 
 	classzone_idx = zone_idx(preferred_zone);
 
+	if (WARN_ON_ONCE(order >= MAX_ORDER))
+		return NULL;
+
 zonelist_scan:
 	/*
 	 * Scan zonelist, looking for a zone with enough free.