ixgbe: clean up Rx time stamping code

Time stamping resources are per-interface so there is no need
to keep separate last_rx_timestamp for each Rx ring, move
last_rx_timestamp to the adapter structure.

With last_rx_timestamp inside adapter, ixgbe_ptp_rx_hwtstamp()
inline function is reduced to a single if statement so it is
no longer necessary. If statement is placed directly in
ixgbe_process_skb_fields() fixing likely/unlikely marking.

Checks for q_vector or adapter to be NULL are superfluous.

Comment about taking I/O hit is a leftover from previous design.

Signed-off-by: Jakub Kicinski <kubakici@wp.pl>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
index c4c526b..d62e7a2 100644
--- a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
+++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
@@ -1664,7 +1664,8 @@
 
 	ixgbe_rx_checksum(rx_ring, rx_desc, skb);
 
-	ixgbe_ptp_rx_hwtstamp(rx_ring, rx_desc, skb);
+	if (unlikely(ixgbe_test_staterr(rx_desc, IXGBE_RXDADV_STAT_TS)))
+		ixgbe_ptp_rx_hwtstamp(rx_ring->q_vector->adapter, skb);
 
 	if ((dev->features & NETIF_F_HW_VLAN_CTAG_RX) &&
 	    ixgbe_test_staterr(rx_desc, IXGBE_RXD_STAT_VP)) {