Follow-up cleanups to the IPv6 fragment extension header support added in
the previous commit:
- Document that minFragBlks is sized for IPv4 but intentionally reused by
decode6 for IPv6 fragments, where it is conservative (IPv6 fragments
carry no per-fragment IP header) and only ever rejects more later
fragments as Unknown, never fewer.
- Add a TestDecode case for a first fragment reachable only through a
chained extension header (base Next Header = Hop-by-Hop Options, which
chains to Fragment). decode6 only parses the Fragment header when it is
the base header's immediate Next Header, so this must classify as
Unknown. The test locks in that scoping decision.
Updates #20083
Updates #20140
Change-Id: Ibece03c6baf2385b0cc399f179819b08cbe921cc
Signed-off-by: James Tucker <james@tailscale.com>
26b2ed0a6a