af_llc: fix if-statement empty body warning

When debugging via dprintk() is not enabled, make the dprintk()
macro be an empty do-while loop, as is done in
<linux/sunrpc/debug.h>.

This fixes a gcc warning when -Wextra is set:
../net/llc/af_llc.c:974:51: warning: suggest braces around empty body in an ‘if’ statement [-Wempty-body]

I have verified that there is not object code change (with gcc 7.5.0).
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: netdev@vger.kernel.org
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/llc/af_llc.c

@@ -47,7 +47,7 @@ static int llc_ui_wait_for_busy_core(struct sock *sk, long timeout);
 #if 0
 #define dprintk(args...) printk(KERN_DEBUG args)
 #else
-#define dprintk(args...)
+#define dprintk(args...) do {} while (0)
 #endif
 
 /* Maybe we'll add some more in the future. */