udp: Do not require sock in udp_tunnel_xmit_skb

The UDP tunnel transmit functions udp_tunnel_xmit_skb and
udp_tunnel6_xmit_skb include a socket argument. The socket being
passed to the functions (from VXLAN) is a UDP created for receive
side. The only thing that the socket is used for in the transmit
functions is to get the setting for checksum (enabled or zero).
This patch removes the argument and and adds a nocheck argument
for checksum setting. This eliminates the unnecessary dependency
on a UDP socket for UDP tunnel transmit.

Signed-off-by: Tom Herbert <therbert@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>