fix(tcp): MPTCP DSS picks 4- vs 8-octet Data ACK/DSN from the value instead of the a/m flags #1215

Describe the bug
transport/tcp.py:2967,2970 set flag_a/flag_m from bit_length() > 32, and Data_MPTCPDSS does not store the a/m flags at all.

To Reproduce
An 8-octet Data ACK carrying a small value, 1e0c2003 0000000000000005, rebuilds as 1e082001 00000005, and the Data Offset drops from 8 to 7.

Expected behavior
The parsed a/m flags are kept and they decide the field width.

Found by the transport round-trip audit (#1202). The repro is from the audit report and I have not re-run it yet.