kernel: fix corrupted padding on small packets with mt753x dsa

Signed-off-by: Felix Fietkau <nbd@nbd.name>
(cherry-picked from commit 203ffc4ca75d43ac32b164c5a474e3ce36999809)

target/linux/generic/pending-5.10/705-net-dsa-tag_mtk-add-padding-for-tx-packets.patch

@@ -0,0 +1,29 @@
+From: Felix Fietkau <nbd@nbd.name>
+Date: Fri, 6 May 2022 21:38:42 +0200
+Subject: [PATCH] net: dsa: tag_mtk: add padding for tx packets
+
+Padding for transmitted packets needs to account for the special tag.
+With not enough padding, garbage bytes are inserted by the switch at the
+end of small packets.
+
+Fixes: 5cd8985a1909 ("net-next: dsa: add Mediatek tag RX/TX handler")
+Signed-off-by: Felix Fietkau <nbd@nbd.name>
+---
+
+--- a/net/dsa/tag_mtk.c
++++ b/net/dsa/tag_mtk.c
+@@ -25,6 +25,14 @@ static struct sk_buff *mtk_tag_xmit(stru
+ 	u8 xmit_tpid;
+ 	u8 *mtk_tag;
+ 
++	/* The Ethernet switch we are interfaced with needs packets to be at
++	 * least 64 bytes (including FCS) otherwise their padding might be
++	 * corrupted. With tags enabled, we need to make sure that packets are
++	 * at least 68 bytes (including FCS and tag).
++	 */
++	if (__skb_put_padto(skb, ETH_ZLEN + MTK_HDR_LEN, false))
++		return NULL;
++
+ 	/* Build the special tag after the MAC Source Address. If VLAN header
+ 	 * is present, it's required that VLAN header and special tag is
+ 	 * being combined. Only in this way we can allow the switch can parse