selftests/bpf: remove xdp_synproxy IP_DF check

In real world production websites, the IP_DF flag
is not always set for each packet from these websites.
the IP_DF flag check breaks Internet connection to
these websites for home based firewall like BPFire
when XDP synproxy program is attached to firewall
Internet facing side interface. see [0]

[0] https://github.com/vincentmli/BPFire/issues/59

Signed-off-by: Vincent Li <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Martin KaFai Lau <[e-mail]>

tools/testing/selftests/bpf/progs/xdp_synproxy_kern.c

···
 
 #define tcp_flag_word(tp) (((union tcp_word_hdr *)(tp))->words[3])
 
-#define IP_DF 0x4000
 #define IP_MF 0x2000
 #define IP_OFFSET 0x1fff
 
···
 		/* TCP doesn't normally use fragments, and XDP can't reassemble
 		 * them.
 		 */
-		if ((hdr->ipv4->frag_off & bpf_htons(IP_DF | IP_MF | IP_OFFSET)) != bpf_htons(IP_DF))
+		if ((hdr->ipv4->frag_off & bpf_htons(IP_MF | IP_OFFSET)) != 0)
 			return XDP_DROP;
 
 		tup.ipv4.saddr = hdr->ipv4->saddr;