mlxsw: spectrum_router: Add support for route replace

Upon the reception of an ENTRY_REPLACE notification, resolve the FIB
node corresponding to the prefix and length and insert the new route
before the first matching entry.

Since the notification also signals the deletion of the replaced route,
delete it from the driver's cache.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>