selftests/net: in rxtimestamp getopt_long needs terminating null entry

[ Upstream commit 865a6cbb2288f8af7f9dc3b153c61b7014fdcf1e ]

getopt_long requires the last element to be filled with zeros.
Otherwise, passing an unrecognized option can cause a segfault.

Fixes: 16e78122 ("selftests/net: Add a test to validate behavior of rx timestamps")
Signed-off-by: Tanner Love <tannerlove@google.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>