fix(ipv6-opts): _make_opt_mpl calls bit_length() on an IPv6Address seed #1185

Describe the bug
_make_opt_mpl calls seed.bit_length() on the parsed seed_id, which is an IPv6Address for this option. Rebuilding the MPL option raises AttributeError: 'IPv6Address' object has no attribute 'bit_length'. The same code is in two places: pcapkit/protocols/internet/hopopt.py:1878 and pcapkit/protocols/internet/ipv6_opts.py:1867.

To Reproduce
Call from_data on the HOPOPT layer of frame #7 of options-ipv6.pcap, or on the IPv6_Opts layer of frame #20. This reproduces on origin/main (20438e3ec).

Impact
Once #1182 lands, these two capture frames raise the error in IPv6.from_data instead of returning wrong bytes.

Found by the #1182 worker. The #1182 cross-review reproduced it independently.