ip6_udp_tunnel: Fix checksum calculation

The UDP checksum calculation for VXLAN tunnels is currently using the
socket addresses instead of the actual packet source and destination
addresses.  As a result the checksum calculated is incorrect in some
cases.

Also uh->check was being set twice, first it was set to 0, and then it is
set again in udp6_set_csum.  This change removes the redundant assignment
to 0.

Fixes: acbf74a7 ("vxlan: Refactor vxlan driver to make use of the common UDP tunnel functions.")

Cc: Andy Zhou <azhou@nicira.com>
Signed-off-by: Alexander Duyck <alexander.h.duyck@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>