net: dsa: Move padding into Broadcom tagger

Instead of having the different master network device drivers
potentially used by DSA/Broadcom tags, move the padding necessary for
the switches to accept short packets where it makes most sense: within
tag_brcm.c. This avoids multiplying the number of similar commits to
e.g: bgmac, bcmsysport, etc.
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dsa/tag_brcm.c

@@ -70,6 +70,18 @@ static struct sk_buff *brcm_tag_xmit_ll(struct sk_buff *skb,
 	if (skb_cow_head(skb, BRCM_TAG_LEN) < 0)
 		return NULL;
 
+	/* The Ethernet switch we are interfaced with needs packets to be at
+	 * least 64 bytes (including FCS) otherwise they will be discarded when
+	 * they enter the switch port logic. When Broadcom tags are enabled, we
+	 * need to make sure that packets are at least 68 bytes
+	 * (including FCS and tag) because the length verification is done after
+	 * the Broadcom tag is stripped off the ingress packet.
+	 *
+	 * Let dsa_slave_xmit() free the SKB
+	 */
+	if (__skb_put_padto(skb, ETH_ZLEN + BRCM_TAG_LEN, false))
+		return NULL;
+
 	skb_push(skb, BRCM_TAG_LEN);
 
 	if (offset)