igc: Refactor Rx timestamp handling

Refactor the Rx timestamp handling in preparation to land
XDP support.

Rx timestamps are put in the Rx buffer by hardware, before the packet
data. When creating the xdp buffer, we will need to check the Rx
descriptor to determine if the buffer contains timestamp information
and consider the offset when setting xdp.data.

The Rx descriptor check is already done in igc_construct_skb(). To
avoid code duplication, this patch moves the timestamp handling to
igc_clean_rx_irq() so both skb and xdp paths can reuse it.

Signed-off-by: Andre Guedes <andre.guedes@intel.com>
Signed-off-by: Vedang Patel <vedang.patel@intel.com>
Signed-off-by: Jithu Joseph <jithu.joseph@intel.com>
Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Tested-by: Dvora Fuxbrumer <dvorax.fuxbrumer@linux.intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>