net/mlx5e: Specify out ifindex when looking up decap route

There is a use case that the local and remote VTEPs are in the same
host. Currently, the out ifindex is not specified when looking up the
decap route for offloads. So in this case, a local route is returned
and the route dev is lo.

Actual tunnel interface can be created with a parameter "dev" [1],
which specifies the physical device to use for tunnel endpoint
communication. Pass this parameter to driver when looking up decap
route for offloads. So that a unicast route will be returned.

[1] ip link add name vxlan1 type vxlan id 100 dev enp4s0f0 remote 1.1.1.1 dstport 4789

Signed-off-by: Chris Mi <cmi@nvidia.com>
Reviewed-by: Roi Dayan <roid@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>