selftests/net: rxtimestamp: Fix an off by one

The > should be >= so that we don't write one element beyond the end of
the array.

Fixes: 16e78122

 ("selftests/net: Add a test to validate behavior of rx timestamps")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>