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 <[e-mail]>
Cc: [e-mail]
Cc: "David S. Miller" <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/llc/af_llc.c

···
 #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. */