As of sam's r175206, arp builds cleanly at WARNS level 6, but the Makefile

was never updated.  Also, clean up the macro that caused the warning in the
first place (no functional changes, just wrapped and reindented).

usr.sbin/arp/Makefile

@@ -4,6 +4,6 @@
 PROG=	arp
 MAN=	arp.4 arp.8
 
-WARNS?=	1
+WARNS?=	6
 
 .include <bsd.prog.mk>

usr.sbin/arp/arp.c

@@ -721,9 +721,13 @@ rtmsg(int cmd, struct sockaddr_inarp *dst, struct sockaddr_dl *sdl)
 	case RTM_GET:
 		rtm->rtm_addrs |= RTA_DST;
 	}
-#define NEXTADDR(w, s) \
-	if ((s) != NULL && rtm->rtm_addrs & (w)) { \
-		bcopy((s), cp, sizeof(*(s))); cp += SA_SIZE(s);}
+#define NEXTADDR(w, s)					   \
+	do {						   \
+		if ((s) != NULL && rtm->rtm_addrs & (w)) { \
+			bcopy((s), cp, sizeof(*(s)));	   \
+			cp += SA_SIZE(s);		   \
+		}					   \
+	} while (0)
 
 	NEXTADDR(RTA_DST, dst);
 	NEXTADDR(RTA_GATEWAY, sdl);