Fix checking if the DF flag was set in ENA driver

The previous way of checking for DF was not valid.
When DF is enabled, the DF bit should be 1.

The original way of checking it was wrong in 2 ways: first of all, it
was not checking for single bit, secondly, it was checking for 0.

Submitted by: Michal Krawczyk <mk@semihalf.com>
Reviewed by: byenduri_gmail.com
Obtained from: Semihalf
Sponsored by: Amazon, Inc.
Differential Revision: https://reviews.freebsd.org/D12865

sys/dev/ena/ena.c

@@ -2640,7 +2640,7 @@ ena_tx_csum(struct ena_com_tx_ctx *ena_tx_ctx, struct mbuf *mbuf)
 	switch (etype) {
 	case ETHERTYPE_IP:
 		ena_tx_ctx->l3_proto = ENA_ETH_IO_L3_PROTO_IPV4;
-		if (ip->ip_off == 0)
+		if ((ip->ip_off & htons(IP_DF)) != 0)
 			ena_tx_ctx->df = 1;
 		break;
 	case ETHERTYPE_IPV6: