ipmr: Make vif fib notifiers common

The fib-notifiers are tightly coupled with the vif_device which is
already common. Move the notifier struct definition and helpers to the
common file; Currently they're only used by ipmr.

Signed-off-by: Yuval Mintz <yuvalm@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>