net: switchdev: merge switchdev_handle_fdb_{add,del}_to_device

To reduce code churn, the same patch makes multiple changes, since they
all touch the same lines:

1. The implementations for these two are identical, just with different
   function pointers. Reduce duplications and name the function pointers
   "mod_cb" instead of "add_cb" and "del_cb". Pass the event as argument.

2. Drop the "const" attribute from "orig_dev". If the driver needs to
   check whether orig_dev belongs to itself and then
   call_switchdev_notifiers(orig_dev, SWITCHDEV_FDB_OFFLOADED), it
   can't, because call_switchdev_notifiers takes a non-const struct
   net_device *.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>