net/mlx5: Introduce alloc_encap and dealloc_encap commands

Implement low-level commands to support vxlan encapsulation.

Signed-off-by: Ilya Lesokhin <ilyal@mellanox.com>
Signed-off-by: Hadar Hen Zion <hadarh@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>