vxlan: Optimize vxlan rcv

vxlan-udp-recv function lookup vxlan_sock struct on every packet
recv by using udp-port number. we can use sk->sk_user_data to
store vxlan_sock and avoid lookup.
I have open coded rcu-api to store and read vxlan_sock from
sk_user_data to avoid sparse warning as sk_user_data is not
__rcu pointer.

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>