SUNRPC: Kill compiler warning when RPC_DEBUG is unset

Loads of these:

linux/net/sunrpc/rpcb_clnt.c:942:2: warning: suggest braces around
  empty body in ‘do’ statement [-Wempty-body]

show up when I unset CONFIG_PROC_SYSCTL.  Seen with

  gcc (GCC) 4.6.1 20110908 (Red Hat 4.6.1-9)
Reported-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

include/linux/sunrpc/debug.h

@@ -74,8 +74,8 @@ extern unsigned int		nlm_debug;
 # define RPC_IFDEBUG(x)		x
 #else
 # define ifdebug(fac)		if (0)
-# define dfprintk(fac, args...)	do ; while (0)
-# define dfprintk_rcu(fac, args...)	do ; while (0)
+# define dfprintk(fac, args...)	do {} while (0)
+# define dfprintk_rcu(fac, args...)	do {} while (0)
 # define RPC_IFDEBUG(x)
 #endif