samples, bpf: Add an explict comment to handle nested vlan tagging.

A codeblock for handling nested vlan trips newbies into thinking it as
duplicate code. Explicitly add a comment to clarify.
Signed-off-by: Muhammad Falak R Wani <falakreyaz@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210809070046.32142-1-falakreyaz@gmail.com

samples/bpf/xdp1_kern.c

@@ -57,6 +57,7 @@ int xdp_prog1(struct xdp_md *ctx)
 
 	h_proto = eth->h_proto;
 
+	/* Handle VLAN tagged packet */
 	if (h_proto == htons(ETH_P_8021Q) || h_proto == htons(ETH_P_8021AD)) {
 		struct vlan_hdr *vhdr;
 
@@ -66,6 +67,7 @@ int xdp_prog1(struct xdp_md *ctx)
 			return rc;
 		h_proto = vhdr->h_vlan_encapsulated_proto;
 	}
+	/* Handle double VLAN tagged packet */
 	if (h_proto == htons(ETH_P_8021Q) || h_proto == htons(ETH_P_8021AD)) {
 		struct vlan_hdr *vhdr;
 

samples/bpf/xdp2_kern.c

@@ -73,6 +73,7 @@ int xdp_prog1(struct xdp_md *ctx)
 
 	h_proto = eth->h_proto;
 
+	/* Handle VLAN tagged packet */
 	if (h_proto == htons(ETH_P_8021Q) || h_proto == htons(ETH_P_8021AD)) {
 		struct vlan_hdr *vhdr;
 
@@ -82,6 +83,7 @@ int xdp_prog1(struct xdp_md *ctx)
 			return rc;
 		h_proto = vhdr->h_vlan_encapsulated_proto;
 	}
+	/* Handle double VLAN tagged packet */
 	if (h_proto == htons(ETH_P_8021Q) || h_proto == htons(ETH_P_8021AD)) {
 		struct vlan_hdr *vhdr;