mlxsw: spectrum_router: Use the nodes list as indication for empty FIB

Currently, each FIB (IPv4 / IPv6) in a virtual router holds a prefix
usage that is used to choose a matching LPM tree, but also to check if
the FIB is empty, so that the LPM tree could be unbound.

Next patches will remove the reliance on the per-FIB prefix usage for
LPM tree matching. Keeping it only to check if the FIB is empty is a
waste, since we can use the nodes ({Prefix, Length}) list instead.

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>