percpu: remove compile warnings caused by __verify_pcpu_ptr()

If percpu pointer is const, __verify_pcpu_ptr() triggers warnings like
the following.

 drivers/net/loopback.c: In function 'loopback_get_stats':
 drivers/net/loopback.c:109: warning: initialization discards qualifiers from pointer target type

Fix it by adding const to the verification target pointer used in
__verify_pcpu_ptr().
Signed-off-by: NTejun Heo <tj@kernel.org>
Reported-by: NStephen Rothwell <sfr@canb.auug.org.au>

include/linux/percpu-defs.h

@@ -24,7 +24,7 @@
  * input parameter is a percpu pointer.
  */
 #define __verify_pcpu_ptr(ptr)	do {					\
-	void __percpu *__vpp_verify = (typeof(ptr))NULL;		\
+	const void __percpu *__vpp_verify = (typeof(ptr))NULL;		\
 	(void)__vpp_verify;						\
 } while (0)