Use v4 default send size as tperf default max_receive_packet_size
Use v4 default send size as tperf default max_receive_packet_size

Summary:
The current default is the smaller one between v4 and v6 default send
sizes. Which means if the tperf test is done on a v4 network, all packets will
be dropped due to lack of receive buffer

Reviewed By: JunqiWang

Differential Revision: D19630955

fbshipit-source-id: 701c1a8728aed00e79297d91460a97af00e4e285

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