mlxsw: spectrum_router: Pass FIB node to LPM tree unlink function

Next patch will try to optimize the LPM tree and make sure only used
prefix lengths are present, to avoid unnecessary look-ups.

Pass the currently removed FIB node to the unlinking function as its
associated prefix length is a potential candidate for removal from the
tree.

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>