Update tcp_lro with tested bugfixes from Netflix and LLNW:

rrs - Lets make the LRO code look for true dup-acks and window update acks
      fly on through and combine.
rrs - Make the LRO engine a bit more aware of ack-only seq space. Lets not
      have it incorrectly wipe out newer acks for older acks when we have
      out-of-order acks (common in wifi environments).
jeggleston - LRO eating window updates

Based on all of the above I think we are RFC compliant doing it this way:

https://tools.ietf.org/html/rfc1122

section 4.2.2.16

"Note that TCP has a heuristic to select the latest window update despite
possible datagram reordering; as a result, it may ignore a window update with
a smaller window than previously offered if neither the sequence number nor the
acknowledgment number is increased."

Submitted by: Kevin Bowling <kevin.bowling@kev009.com>
Reviewed by: rstone gallatin
Sponsored by: NetFlix and Limelight Networks
Differential Revision: https://reviews.freebsd.org/D14540