net: Simplify GRE case in flow_dissector

Do break when we see routing flag or a non-zero version number in GRE
header.

Acked-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>