net: drop_monitor: make last_rx timestamp private

It was recently pointed out to me that the last_rx field of the
net_device structure wasn't updated regularly.  In fact only the
bonding driver really uses it currently.  Since the drop_monitor code
relies on the last_rx field to detect drops on recevie in hardware, We
need to find a more reliable way to rate limit our drop checks (so
that we don't check for drops on every frame recevied, which would be
inefficient.  This patch makes a last_rx timestamp that is private to
the drop monitor code and is updated for every device that we track.

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>