mlxsw: spectrum_router: Return an error for non-default FIB rules

Since commit 9776d325

 ("net: Move call_fib_rule_notifiers up in
fib_nl_newrule") it is possible to forbid the installation of
unsupported FIB rules.

Have mlxsw return an error for non-default FIB rules in addition to the
existing extack message.

Example:
# ip rule add from 198.51.100.1 table 10
Error: mlxsw_spectrum: FIB rules not supported.

Note that offload is only aborted when non-default FIB rules are already
installed and merely replayed during module initialization.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>