vxlan: clean up rx error path

When there are unrecognized flags present in the vxlan header, it doesn't
make much sense to return the packet for further UDP processing, especially
considering that for other invalid flag combinations we drop the packet
because of previous checks.

This means we return positive value only at the beginning of the function
where tun_dst is not yet allocated. This allows us to get rid of the
bad_flags and error jump labels.

When we're dropping packet, we need to free tun_dst now.

Signed-off-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>