vxlan: Allow vetoing of FDB notifications

Change vxlan_fdb_switchdev_call_notifiers() to return the result from
calling switchdev notifiers. Propagate the error number up the stack.

In vxlan_fdb_update_existing() and vxlan_fdb_update_create() add
rollbacks to clean up the work that was done before the veto.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>