MFC r360479: Prevent premature shrinking of the scaled receive window

which can cause a TCP client to use invalid or stale TCP sequence numbers for ACK packets.

Packets with old sequence numbers are ignored and not used to update the send window size.
This might cause the TCP session to hang indefinitely under some circumstances.

Reported by: Cui Cheng
Reviewed by: tuexen (mentor), rgrimes (mentor, blanket)
Approved by: tuexen (mentor), rgrimes (mentor, blanket)
MFC after: 3 weeks
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D24515