vxlan: Eliminate dependency on UDP socket in transmit path

In the vxlan transmit path there is no need to reference the socket
for a tunnel which is needed for the receive side. We do, however,
need the vxlan_dev flags. This patch eliminate references
to the socket in the transmit path, and changes VXLAN_F_UNSHAREABLE
to be VXLAN_F_RCV_FLAGS. This mask is used to store the flags
applicable to receive (GBP, CSUM6_RX, and REMCSUM_RX) in the
vxlan_sock flags.

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