Fix LWM2M header calculation in lwm2m_engine.c

[zephyrbot]

Reported by Michael Scott:

Adding this JIRA to track fixing the way LWM2M engine calculates the header length of the LWM2M UDP packet in subsys/net/lib/lwm2m/lwm2m_engine.c:2179

(Imported from Jira ZEP-2413)

[zephyrbot]

by Michael Scott:

Jukkar commented during code review:
This is dangerous but will work most of the time. But if the header does not fit into first fragment, then this will fail miserably.

The fix might require some changes in zoap too so we can do this a bit later, just do not forget about this. BTW, the start of the CoAP message could be figured out like this:

frag = net_frag_get_pos(pkt,
net_pkt_ip_hdr_len(pkt) +
net_pkt_ipv6_ext_len(pkt) +
sizeof(struct net_udp_hdr),
&offset);

The offset will then tell how many bytes from frag->data the coap header is.

[mike-scott]

The code this issue is referencing is near the top of lwm2m_udp_receive() in lwm2m_engine.c:
/*
* zoap expects that buffer->data starts at the
* beginning of the CoAP header
*/
header_len = net_pkt_appdata(pkt) - pkt->frags->data;
net_buf_pull(pkt->frags, header_len);

[mike-scott]

This has since been fixed in the following commit: fb734ac#diff-88ec0070db0157cb49aeb6609e210f1aL2598