flow_dissector: Fix GRE header error path

Now, when an unexpected element in the GRE header appears, we break so
the l4 ports are processed. But since the ports are processed
unconditionally, there will be certainly random values dissected. Fix
this by just bailing out in such situations.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>