net: ethernet: enetc: do not always access skb_shared_info in the XDP path

Move XDP skb_shared_info structure initialization in from
enetc_map_rx_buff_to_xdp() to enetc_add_rx_buff_to_xdp() and do not always
access skb_shared_info in the xdp_buff/xdp_frame since it is located in a
different cacheline with respect to hard_start and data xdp pointers.
Rely on XDP_FLAGS_HAS_FRAGS flag to check if it really necessary to access
non-linear part of the xdp_buff/xdp_frame.

Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Tested-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>