odhcpd: ensure zero padding on DNSSL

[efahl]

From https://www.rfc-editor.org/rfc/rfc8106#section-5.2 regarding the DNSSL field:

Because the size of this field MUST be a multiple of
8 octets, for the minimum multiple including the domain
name representations, the remaining octets other than the
encoding parts of the domain name representations MUST be
padded with zeros.

The current code leaves the trailing octets in uninitialized state, resulting in parsing errors on (at least) systemd RA clients. This commit restores the explicit zeroing of the trailing octets.

Fixes: 0a54ce0
Fixes: openwrt/openwrt#22351

---

Tested on x86/64 snapshot:

$ tcpdump -v -n -p 'icmp6[icmp6type] == icmp6-routeradvert' -i br-lan -X -s0
tcpdump: listening on br-lan, link-type EN10MB (Ethernet), snapshot length 262144 bytes
06:52:22.514144 IP6 (flowlabel 0x6e2ab, hlim 255, next-header ICMPv6 (58), payload length 256) fe80::7e2b:e1ff:fe13:73a8 > ff02::1: [icmp6 sum ok] ICMP6, router advertisement, length 256
...
          dnssl option (31), length 24 (3):  lifetime 5163s, domain(s): xxx.lan.
...
        0x0100:                      1f03 0000 0000 142b  ...............+
        0x0110:  0378 7878 036c 616e 0000 0000 0000 0000  .xxx.lan........

[efahl]

@hauke
This might be important for 25.12, as it caused issues for me on at least 3 Ubuntu servers (2x 22.04, 1x 24.04) which could not get IPv6 addresses due to systemd failing to parse the RAs when using local domain names.

[efahl]

As part of my preliminary investigation, I did a sanity check to make sure I understood where things were going by writing 0xdd instead of zeros into the DNSSL block. Here's what I saw on the test device:

dnssl option (31), length 24 (3):  lifetime 4727s, domain(s): wang.lan. <BAD PTR> [|icmp6]

0x0100:  0000 0000 0000 0001 1f03 0000 0000 1277  ...............w
0x0110:  0477 616e 6703 6c61 6e00 dddd dddd dddd  .wang.lan.......
0x0120:  0701 0000 0009 27c0                      ......'.

Note that with this failure to parse the DNSSL block, that the final advertisement interval option (7), length 8 (1): 600000ms block is ignored by tcpdump. I assume that systemd is doing likewise...

[Noltari]

Merged, thanks @efahl!