geneve: handle ipv6 priority like ipv4 tos

Other callers of udp_tunnel6_xmit_skb just pass 0 for the prio
argument.  Jesse Gross <jesse@nicira.com> suggested that prio is really
the same as IPv4's tos and should be handled the same, so this is my
interpretation of that suggestion.

Signed-off-by: John W. Linville <linville@tuxdriver.com>
Reported-by: Jesse Gross <jesse@nicira.com>
Reviewed-by: Jesse Gross <jesse@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>