Make get_order(0) return 0

This is needed for some followup patches. Some old drivers 
(like xd.c) pass 0 to get_order and the compat wrapper for 
the mask allocator doesn't like the resulting underflow.

Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Andi Kleen <andi@firstfloor.org>

---
 include/asm-generic/page.h |    3 +++
 1 file changed, 3 insertions(+)

Index: linux/include/asm-generic/page.h
===================================================================
--- linux.orig/include/asm-generic/page.h
+++ linux/include/asm-generic/page.h
@@ -11,6 +11,9 @@ static __inline__ __attribute_const__ in
 {
 	int order;
 
+	if (size == 0)
+		return 0;
+
 	size = (size - 1) >> (PAGE_SHIFT - 1);
 	order = -1;
 	do {