net/mlx5e: Call vxlan_get_rx_port() with rtnl lock

Hold the rtnl lock when calling vxlan_get_rx_port().

Fixes: b7aade15

 ("vxlan: break dependency with netdev drivers")
Signed-off-by: Matthew Finlay <matt@mellanox.com>
Reported-by: Alexander Duyck <alexander.duyck@gmail.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>