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>