ntp: don't send packets with RX/TX timestamp equal to another timestamp

ntp: don't send packets with RX/TX timestamp equal to another timestamp

Before sending a new packet, check if the receive/transmit timestamp
is not equal to the origin timestamp or the previous receive/transmit
timestamp in order to prevent the packet from being its own valid
response (in the symmetric mode) and invalidate responses to the
previous packet.

This improves protection against replay attacks in the symmetric mode.