net: dsa: tag_edsa: let DSA core deal with TX reallocation

Now that we have a central TX reallocation procedure that accounts for
the tagger's needed headroom in a generic way, we can remove the
skb_cow_head call.

Note that the VLAN code path needs a smaller extra headroom than the
regular EtherType DSA path. That isn't a problem, because this tagger
declares the larger tag length (8 bytes vs 4) as the protocol overhead,
so we are covered in both cases.

Cc: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>