mlxsw: core: Add support for using SKB control buffer

The SKB control buffer is useful (and used) for bookkeeping of information
related to that SKB. Add helpers so that the mlxsw driver(s) can safely use
the buffer as well. The structure is currently empty, individual users will
add members to it as necessary.

Note that SKB allocation functions already clear the buffer, so the cleanup
is only necessary when ndo_start_xmit is called.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>