vxlan: Add a function to init switchdev_notifier_vxlan_fdb_info

There are currently two places that need to initialize the notifier info
structure, and one more is coming next when vxlan_fdb_replay() is
introduced. These three instances have / will have very similar code
that is easy to abstract away into a named function.

Add such function, vxlan_fdb_switchdev_notifier_info(), and call it from
vxlan_fdb_switchdev_call_notifiers() and vxlan_fdb_find_uc().

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