tcp/lro: Allow drivers to set the TCP ACK/data segment aggregation limit

ACK aggregation limit is append count based, while the TCP data segment
aggregation limit is length based. Unless the network driver sets these
two limits, it's an NO-OP.

Reviewed by: adrian, gallatin (previous version), hselasky (previous version)
Approved by: adrian (mentor)
MFC after: 1 week
Sponsored by: Microsoft OSTC
Differential Revision: https://reviews.freebsd.org/D5185