net/mlx5e: vxlan: Use RCU for vxlan table lookup

Remove the spinlock protecting the vxlan table and use RCU instead.
This will improve performance as it will eliminate contention on data
path cores.

Fixes: b3f63c3d

 ("net/mlx5e: Add netdev support for VXLAN tunneling")
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Reviewed-by: Maxim Mikityanskiy <maximmi@mellanox.com>