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