net: dsa: ensure linearized SKBs in case of tail taggers

The function skb_put() that is used by tail taggers to make room for the
DSA tag must only be called for linearized SKBS. However in case that the
slave device inherited features like NETIF_F_HW_SG or NETIF_F_FRAGLIST the
SKB passed to the slaves transmit function may not be linearized.
Avoid those SKBs by clearing the NETIF_F_HW_SG and NETIF_F_FRAGLIST flags
for tail taggers.
Furthermore since the tagging protocol can be changed at runtime move the
code for setting up the slaves features into dsa_slave_setup_tagger().

Suggested-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Lino Sanfilippo <LinoSanfilippo@gmx.de>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>