x86: Use correct byte-sized register constraint in __add()

Similar to:

 2ca052a x86: Use correct byte-sized register constraint in __xchg_op()

... the __add() macro also needs to use a "q" constraint in the
byte-sized case, lest we try to generate an illegal register.

Link: http://lkml.kernel.org/r/4F7A3315.501@goop.org
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: Leigh Scott <leigh123linux@googlemail.com>
Cc: Thomas Reitmayr <treitmayr@devbase.at>
Cc: <stable@vger.kernel.org> v3.3

arch/x86/include/asm/cmpxchg.h

@@ -173,7 +173,7 @@ extern void __add_wrong_size(void)
 		switch (sizeof(*(ptr))) {				\
 		case __X86_CASE_B:					\
 			asm volatile (lock "addb %b1, %0\n"		\
-				      : "+m" (*(ptr)) : "ri" (inc)	\
+				      : "+m" (*(ptr)) : "qi" (inc)	\
 				      : "memory", "cc");		\
 			break;						\
 		case __X86_CASE_W:					\