MFC 324050

MFC 324050

    tcp: Don't "negotiate" MSS.

    _NO_ OSes actually "negotiate" MSS.

    RFC 879:
    "... This Maximum Segment Size (MSS) announcement (often mistakenly
    called a negotiation) ..."

    This negotiation behaviour was introduced 11 years ago by r159955
    without any explaination about why FreeBSD had to "negotiate" MSS:

        In syncache_respond() do not reply with a MSS that is larger than what
        the peer announced to us but make it at least tcp_minmss in size.

        Sponsored by:   TCP/IP Optimization Fundraise 2005

    The tcp_minmss behaviour is still kept.

    Syncookie fix was prodded by tuexen, who also helped to test this
    patch w/ packetdrill.

    Reviewed by:    tuexen, karels, bz (previous version)
    Sponsored by:   Microsoft
    Differential Revision:  https://reviews.freebsd.org/D12430