mlxsw: spectrum_router: Don't rely on missing extack to symbolize dump

Currently if info->extack is NULL, mlxsw assumes that the event came
down from dump. Originally, the dump did not propagate the return value
back to the original caller (fib_notifier_register()). However, that is
now happening. So benefit from this and push the error up if it happened.
Remove rule cases in work handlers that are now dead code.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>