fix(link): L2TPv2 drops the reserved flag bits on rebuild #1207

Describe the bug
The flags BitField at schema/link/l2tp.py:45 defines no reserved subfields, and neither does Data_Flags (l2tpv2.py:203) or the flags dict that make builds. Reserved bits 2, 3, 5 and 8-11 are lost.

To Reproduce
Parse 200200010002ff03002145000000 and rebuild it with from_data. The result is 000200010002ff03002145000000. Masks 0x1000, 0x0400 and 0x00f0 behave the same way, including inside UDP on port 1701.

Expected behavior
The reserved bits round-trip unchanged.

Found by the link round-trip audit (#1202) and re-run by me on main at 630f0a581.