mlxsw: spectrum_router: Abort on source-specific routes

Without resorting to ACLs, the device performs route lookup solely based
on the destination IP address.

In case source-specific routing is needed, an error is returned and the
abort mechanism is activated, thus allowing the kernel to take over
forwarding decisions.

Instead of aborting, we can trap specific destination prefixes where
source-specific routes are present, but this will result in a lot more
code that is unlikely to ever be used.

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>