fix(l2tp): L2TPv2.from_data drops the L flag and the Length field #1172

Describe the bug
L2TPv2.from_data drops the length-present (L) flag and the Length field, so a header that carries a Length does not round-trip.

To Reproduce
On main 748c10dce:

from pcapkit.protocols.link.l2tpv2 import L2TPv2
raw = bytes.fromhex('c802000c0001000000010002')   # ZLB ACK: T, L and S set, length=12
L2TPv2.from_data(L2TPv2(raw).info).data.hex()    # '88020001000000010002'

The parse reads length=12 with len=True, seq=True. The rebuild clears L (c8 → 88) and omits the 2-octet Length field.

Expected behavior
from_data(x.info).data == x.data. That means _make_data should carry the L flag and the length through to make(), as #1166 did for IPv4 and UDP.

Additional context
Found by the #1171 worker and confirmed by its cross-review. I re-ran the reproduction above myself.