ixgbe: workaround errata about UDP frames with zero checksum

Intel 82599 has errata related to IPv4 UDP frames with zero checksum.
It reports such datagrams with L4 integrity errors in IXGBE_XEC
register. And after afb1aa4 commit such errors are reported
via IFCOUNTER_IERRORS. This confuses users, since actually all frames
are handled correctly by the system.
To workaround the problem, let's ignore the XEC register value for
82599 cards for now.

PR:		266048
Discussed with:	erj
MFC after:	1 week
Sponsored by:	Yandex LLC

(cherry picked from commit 8526120)

sys/dev/ixgbe/if_ix.c

@@ -1584,8 +1584,12 @@ ixgbe_update_stats_counters(struct ixgbe_softc *sc)
 	 * - fragmented packets count,
 	 * - oversized packets count,
 	 * - jabber count.
+	 *
+	 * Ignore XEC errors for 82599 to workaround errata about
+	 * UDP frames with zero checksum.
 	 */
-	IXGBE_SET_IERRORS(sc, stats->crcerrs + stats->illerrc + stats->xec +
+	IXGBE_SET_IERRORS(sc, stats->crcerrs + stats->illerrc +
+	    (hw->mac.type != ixgbe_mac_82599EB ? stats->xec : 0) +
 	    stats->mpc[0] + stats->rlec + stats->ruc + stats->rfc + stats->roc +
 	    stats->rjc);
 } /* ixgbe_update_stats_counters */