vxlan: do not set SKB_GSO_UDP

Since SKB_GSO_* flags are set by appropriate gso_segment callback
in TCP/UDP layer.

CC: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: Dmitry Kravkov <dmitry@broadcom.com>
Signed-off-by: Eilon Greenstein <eilong@broadcom.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>