net/mlx5e: Refactor VXLAN tunnel encap offloading code

Separates the vxlan header encap logic from the general ipv4/6
encapsulation methods, thus allowing the common IP encap/decap code to
branch in downstream patch to multiple IP tunnels.

Code refactoring with no functional change.

Signed-off-by: Oz Shlomo <ozsh@mellanox.com>
Reviewed-by: Eli Britstein <elibr@mellanox.com>
Reviewed-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>