selftests: txtimestamp: add new command-line flags.

A longer sleep duration between sendmsg()s makes more cachelines to be
evicted and results in higher latency. Making the duration configurable.

Add the following new flags:
-S: Configurable sleep duration.
-b: Busy loop instead of poll().

Remove the following flag:
-D: No delay between packets: subsumed by -S.

Signed-off-by: Jian Yang <jianyang@google.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>