net_sched: fix unused variable warning in stmmac

The new tcf_exts_for_each_action() macro doesn't reference its
arguments when CONFIG_NET_CLS_ACT is disabled, which leads to
a harmless warning in at least one driver:

drivers/net/ethernet/stmicro/stmmac/stmmac_tc.c: In function 'tc_fill_actions':
drivers/net/ethernet/stmicro/stmmac/stmmac_tc.c:64:6: error: unused variable 'i' [-Werror=unused-variable]

Adding a cast to void lets us avoid this kind of warning.
To be on the safe side, do it for all three arguments, not
just the one that caused the warning.

Fixes: 244cd96adb5f ("net_sched: remove list_head from tc_action")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/pkt_cls.h

@@ -303,7 +303,7 @@ static inline void tcf_exts_put_net(struct tcf_exts *exts)
 	for (i = 0; i < TCA_ACT_MAX_PRIO && ((a) = (exts)->actions[i]); i++)
 #else
 #define tcf_exts_for_each_action(i, a, exts) \
-	for (; 0; )
+	for (; 0; (void)(i), (void)(a), (void)(exts))
 #endif
 
 static inline void