Enable filter functionality for DLT_DSA_TAG_DSA and DLT_DSA_TAG_EDSA.

[Hexxel]

We want to use tcpdump filter functionality on a DSA/EDSA link layer device, but there doesn't seem to be supported right now. I'm not well versed in the libpcap implementation so I naively looked at the existing code and did the following changes in this PR. Please don't hit me over the head :/
Is this the right way of doing it or do you have any suggestions where I should look for a better solution/approach?

Best regards,

[Hexxel]

No actions on this PR?

[infrastation]

FWIW, I do not see any immediate problems with the proposed change.

Before:

$ ./tcpdump -n -r tests/dsa.pcap -ve icmp
reading from file tests/dsa.pcap, link-type DSA_TAG_DSA (Marvell DSA), snapshot length 262144
tcpdump: Marvell DSA link-layer type filtering not implemented

After:

$ ./tcpdump -n -r tests/dsa.pcap -ve -c 1 'icmp[icmptype] = icmp-echo'
reading from file tests/dsa.pcap, link-type DSA_TAG_DSA (Marvell DSA), snapshot length 262144
23:21:39.544060 00:50:b6:29:10:70 > d6:c5:28:21:3e:af, Marvell DSA mode Forward, dev 0, port 1, untagged, VID 0, FPri 0, ethertype IPv4 (0x0800), length 102: (tos 0x0, ttl 64, id 14057, offset 0, flags [DF], proto ICMP (1), length 84)
    192.168.30.1 > 192.168.30.2: ICMP echo request, id 13586, seq 1, length 64

$ ./tcpdump -n -r tests/edsa.pcap -ve -c 1 'arp'
reading from file tests/edsa.pcap, link-type DSA_TAG_EDSA (Marvell EDSA), snapshot length 262144
23:21:49.680084 c6:e8:9f:7d:69:da > 00:50:b6:29:10:7e, Marvell EDSA ethertype 0xdada (Unknown), rsvd 0 0, mode From CPU, target dev 0, port 0, untagged, VID 0, FPri 0, ethertype ARP (0x0806), length 50: Ethernet (len 6), IPv4 (len 4), Request who-has 192.168.20.1 tell 192.168.20.2, length 28

[infrastation]

Any objections to merge this?

[infrastation]

This will need a change log entry. A few filter tests would be useful too.

[infrastation]

If these changes are applied and off_nl_nosnap is set to OFFSET_NOT_SET and the DLT contains an Ethernet packet (with a commensurate switch tag) that is a 802.2 LLC packet, will everything work as usual?

[Hexxel]

I really don't know, sorry.

[infrastation]

Alright, let's think about it.

[infrastation]

As it turns out, LLC indeed may be present and there is more code paths to take into account. Commit 588655e implements DLT_DSA_TAG_DSA filtering and adds tests.