mlxsw: spectrum_trap: Push Ethernet header before reporting trap

devlink maintains packets and bytes statistics for each trap. Since
eth_type_trans() was called to set the skb's protocol, the data pointer
no longer points to the start of the packet and the bytes accounting is
off by 14 bytes.

Fix this by pushing the skb's data pointer to the start of the packet.

Fixes: b5ce611f ("mlxsw: spectrum: Add devlink-trap support")
Reported-by: Alex Kushnarov <alexanderk@mellanox.com>
Tested-by: Alex Kushnarov <alexanderk@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>