net/mlx5e: Extract tc tunnel encap/decap code to dedicated file

Following patches in series extend the extracted code with routing
infrastructure. To improve code modularity created a dedicated
tc_tun_encap.c source file and move encap/decap related code to the new
file. Export code that is used by both regular TC code and encap/decap code
into tc_priv.h (new header intended to be used only by TC module). Rename
some exported functions by adding "mlx5e_" prefix to their names.

Signed-off-by: Vlad Buslov <vladbu@nvidia.com>
Signed-off-by: Dmytro Linkin <dlinkin@nvidia.com>
Reviewed-by: Roi Dayan <roid@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>