mlxsw: spectrum_router: Remove FIB info from FIB entry struct

After the previous changes, the FIB info is embedded in every nexthop
group struct, which in turn is embedded in every FIB entry struct.

We can therefore safely remove the FIB info from the entry struct. This
has the added advantage of making the router-related structs more
generic and suitable for use with IPv6 offloads.

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>