igc: Fix HW RX timestamp when passed by ZC XDP

Fixes HW RX timestamp in the following scenario:
- AF_PACKET socket with enabled HW RX timestamps is created
- AF_XDP socket with enabled zero copy is created
- frame is forwarded to the BPF program, where the timestamp should
  still be readable (extracted by igc_xdp_rx_timestamp(), kfunc
  behind bpf_xdp_metadata_rx_timestamp())
- the frame got XDP_PASS from BPF program, redirecting to the stack
- AF_PACKET socket receives the frame with HW RX timestamp

Moves the skb timestamp setting from igc_dispatch_skb_zc() to
igc_construct_skb_zc() so that igc_construct_skb_zc() is similar to
igc_construct_skb().

This issue can also be reproduced by running:
 # tools/testing/selftests/bpf/xdp_hw_metadata enp1s0
When a frame with the wrong port 9092 (instead of 9091) is used:
 # echo -n xdp | nc -u -q1 192.168.10.9 9092
then the RX timestamp is missing and xdp_hw_metadata prints:
 skb hwtstamp is not found!

With this fix or when copy mode is used:
 # tools/testing/selftests/bpf/xdp_hw_metadata -c enp1s0
then RX timestamp is found and xdp_hw_metadata prints:
 found skb hwtstamp = 1736509937.852786132

Fixes: 069b142f ("igc: Add support for PTP .getcyclesx64()")
Signed-off-by: Zdenek Bouska <zdenek.bouska@siemens.com>
Acked-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Florian Bezdeka <florian.bezdeka@siemens.com>
Reviewed-by: Song Yoong Siang <yoong.siang.song@intel.com>
Tested-by: Mor Bar-Gabay <morx.bar.gabay@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>