net: dcb: modify dcb_app_add to take list_head ptr as parameter

In preparation to DCB rewrite. Modify dcb_app_add to take new struct
list_head * as parameter, to make the used list configurable. This is
done to allow reusing the function for adding rewrite entries to the
rewrite table, which is introduced in a later patch.

Signed-off-by: Daniel Machon <daniel.machon@microchip.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>