netem: loss model API sizes

The new netem loss model is configured with nested netlink messages.
This code is being overly strict about sizes, and is easily confused
by padding (or possible future expansion). Also message
for gemodel is incorrect.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>