net: enetc: save the parsed information of PTP packet to skb->cb

net: enetc: save the parsed information of PTP packet to skb->cb

Currently, the Tx PTP packets are parsed twice in the enetc driver, once
in enetc_xmit() and once in enetc_map_tx_buffs(). The latter is duplicate
and is unnecessary, since the parsed information can be saved to skb->cb
so that enetc_map_tx_buffs() can get the previously parsed data from
skb->cb. Therefore, add struct enetc_skb_cb as the format of the data
in the skb->cb buffer to save the parsed information of PTP packet. Use
saved information in enetc_map_tx_buffs() to avoid parsing data again.

In addition, rename variables offset1 and offset2 in enetc_map_tx_buffs()
to corr_off and tstamp_off for better readability.

Signed-off-by: Wei Fang <wei.fang@nxp.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Link: https://patch.msgid.link/20250829050615.1247468-10-wei.fang@nxp.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>