selftests: net: cmsg_sender: support Tx timestamping

Support requesting Tx timestamps:

 $ ./cmsg_sender -p i -t -4 $tgt 123 -d 1000
 SCHED ts0 61us
   SND ts0 1071us

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>