inet6: parse only complete Routing Header addresses

[KernelClint]

An IPv6 Routing Header carries a list of addresses, and its len field counts the header's size in
eight-byte units. The generic form parses that list at
scapy/layers/inet6.py:997-1007:

length_from=lambda pkt: 8 * pkt.len

Addresses are sixteen bytes, so an odd len describes a whole number of eight-byte units but only
half an address. The field at scapy/layers/inet6.py:252-271 converts each chunk it is given into
an address, including a final incomplete one, and parsing raises partway through. The packet ends
as IPv6 followed by unparsed data, and anything reading the layer chain — a bridge_and_sniff()
transform, for instance — does not see the transport header that a conforming receiver does.

The change parses only the complete entries and keeps an odd trailing eight bytes as generic
routing data:

-                                length_from=lambda pkt: 8 * pkt.len)]
+                                length_from=lambda pkt: 16 * (pkt.len // 2)),
+                   ConditionalField(
+                       StrLenField("data", b"", length_from=lambda pkt: 8),
+                       lambda pkt: (bool(pkt.getfieldval("data"))
+                                    if pkt.len is None else pkt.len % 2),
+                   )]

The extra field is conditional on an odd length, so even-length Routing Headers keep exactly the
representation and build output they have today. The length adjustment is updated to match so a
header built with trailing data round-trips.

The added regression parses an odd-length header and asserts it yields an empty address list, the
trailing data, and the transport header beneath it. With only the test, the IPv6 suite reports 522
passed and 1 failed; with the source change, 523 passed and 0 failed.

Performance was measured on one computer, before and after the fix: parsing a valid even-length
header took 6,777.1 ns before and 6,574.2 ns after. Repeat runs of this test moved by about 8%,
which is wide enough that only a large change would show — this is not one.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.64%. Comparing base (b3bbcc8) to head (013d9ec).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5142   +/-   ##
=======================================
  Coverage   80.63%   80.64%           
=======================================
  Files         390      390           
  Lines       96936    96936           
=======================================
+ Hits        78168    78176    +8     
+ Misses      18768    18760    -8     

Files with missing lines	Coverage Δ
scapy/layers/inet6.py	88.71% <ø> (ø)

... and 7 files with indirect coverage changes

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