percpu: Avoid extra NOP in percpu_cmpxchg16b_double

percpu_cmpxchg16b_double() uses alternative_io() and looks like :

e8 .. .. .. ..  call this_cpu_cmpxchg16b_emu
X bytes	    NOPX

or, once patched (if cpu supports native instruction) on SMP build :

65 48 0f c7 0e  cmpxchg16b %gs:(%rsi)
0f 94 c0        sete %al

on !SMP build :

48 0f c7 0e     cmpxchg16b (%rsi)
0f 94 c0        sete %al

Therefore, NOPX should be :

P6_NOP3 on SMP
P6_NOP2 on !SMP
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Acked-by: Christoph Lameter <cl@linux.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Pekka Enberg <penberg@kernel.org>
Signed-off-by: Tejun Heo <tj@kernel.org>

arch/x86/include/asm/percpu.h

@@ -509,6 +509,11 @@ do {									\
  * it in software.  The address used in the cmpxchg16 instruction must be
  * aligned to a 16 byte boundary.
  */
+#ifdef CONFIG_SMP
+#define CMPXCHG16B_EMU_CALL "call this_cpu_cmpxchg16b_emu\n\t" P6_NOP3
+#else
+#define CMPXCHG16B_EMU_CALL "call this_cpu_cmpxchg16b_emu\n\t" P6_NOP2
+#endif
 #define percpu_cmpxchg16b_double(pcp1, o1, o2, n1, n2)			\
 ({									\
 	char __ret;							\
@@ -517,7 +522,7 @@ do {									\
 	typeof(o2) __o2 = o2;						\
 	typeof(o2) __n2 = n2;						\
 	typeof(o2) __dummy;						\
-	alternative_io("call this_cpu_cmpxchg16b_emu\n\t" P6_NOP4,	\
+	alternative_io(CMPXCHG16B_EMU_CALL,				\
 		       "cmpxchg16b " __percpu_prefix "(%%rsi)\n\tsetz %0\n\t",	\
 		       X86_FEATURE_CX16,				\
 		       ASM_OUTPUT2("=a"(__ret), "=d"(__dummy)),		\