vxlan: consolidate csum flag handling

The flag for tx checksumming for tunneling over IPv4 and IPv6 is different.
Decide whether to do tx checksumming in vxlan_xmit_one and pass it on as
a separate flag. This will allow for tx path consolidation in the next
patch.

Unfortunately, gcc is not clever enough to see that udp_sum is always
initialized and gives an uninitialized variable warning. Set it to false to
silence the warning.

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