rxperf: Really set UDP buffer size

rxperf: Really set UDP buffer size

On Unix, the UDP buffer size has to be set before we open any sockets.
Otherwise, the default (64k) buffer size is used and never changed.

Move the calls to SetUdpBufSize in rxperf.c so that the -u command
line option actually has an effect.

Change-Id: Ic925faabca8264c0359693e97c2ab5b637902797
Reviewed-on: http://gerrit.openafs.org/2855
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>