net: mvpp2: prevent buffer overflow in mvpp22_rss_ctx()

The "rss_context" variable comes from the user via  ethtool_get_rxfh().
It can be any u32 value except zero.  Eventually it gets passed to
mvpp22_rss_ctx() and if it is over MVPP22_N_RSS_TABLES (8) then it
results in an array overflow.

Fixes: 895586d5 ("net: mvpp2: cls: Use RSS contexts to handle RSS tables")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/marvell/mvpp2/mvpp2_main.c

@@ -4329,6 +4329,8 @@ static int mvpp2_ethtool_get_rxfh_context(struct net_device *dev, u32 *indir,
 
 	if (!mvpp22_rss_is_supported())
 		return -EOPNOTSUPP;
+	if (rss_context >= MVPP22_N_RSS_TABLES)
+		return -EINVAL;
 
 	if (hfunc)
 		*hfunc = ETH_RSS_HASH_CRC32;