This patch was submitted by Richard Russo <russor whatsapp.com>.
The problem is that some bad gateway can respond with ICMP need frag suggesting the same MTU we already use. This has been seen in the wild. Theoretically there could be response with larger MTU. Our stack reacts on such ICMP resending the whole TCP window again, for each ICMP received. I believe this isn't a best behaviour. Richard suggest to ignore such ICMPs. His patch actually modifies tp_maxseg, it just suppresses resend of the window. This covers properly case of equal MTU offer. I suppose, that to cover case of growing offer, we should also add protection against increasing tp_maxseg.