mlxsw: spectrum_router: Set offload_failed flag

When FIB_EVENT_ENTRY_{REPLACE, APPEND} are triggered and route insertion
fails, FIB abort is triggered.

After aborting, set the appropriate hardware flag to make the kernel emit
RTM_NEWROUTE notification with RTM_F_OFFLOAD_FAILED flag.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>