When processing ICMP need frag message, ignore the suggested MTU unless it
is smaller than the current one for this connection. This is behavior
specified by RFC 1191, and this is how original BSD stack behaved, but this
was unintentionally regressed in r182851.

Reported & tested by: Richard Russo <russor whatsapp.com>
Differential Revision: D3567
Sponsored by: Nginx, Inc.