net: fou: remove redundant code in gue_udp_recv

Remove not useful protocol version check in gue_udp_recv since just
gue version 0 can hit that code. Moreover remove duplicated hdrlen
computation

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>