vxlan: Fix boolean flip in VXLAN_F_UDP_ZERO_CSUM6_[TX|RX]

In "vxlan: Call udp_sock_create" there was a logic error that resulted in
the default for IPv6 VXLAN tunnels going from using checksums to not using
checksums.  Since there is currently no support in iproute2 for setting
these values it means that a kernel after the change cannot talk over a IPv6
VXLAN tunnel to a kernel prior the change.

Fixes: 3ee64f39

 ("vxlan: Call udp_sock_create")

Cc: Tom Herbert <therbert@google.com>
Signed-off-by: Alexander Duyck <alexander.h.duyck@redhat.com>
Acked-by: Tom Herbert <therbert@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>