mlxsw: spectrum_router: Use the available router pointer for netevent handling

This code handles NETEVENT_DELAY_PROBE_TIME_UPDATE, which is invoked every
time the delay_probe_time changes. mlxsw router currently only maintains
one timer, so the last delay_probe_time set wins.

Currently, mlxsw uses mlxsw_sp_port_lower_dev_hold() to find a reference to
the router. This is no longer necessary. But as a side effect, this makes
sure that only updates to "interesting netdevices" (ones that have a
physical netdevice lower) are projected.

Retain that side effect by calling mlxsw_sp_port_dev_lower_find_rcu() and
punting if there is none. Then just proceed using the router pointer that's
already at hand in the helper.

Note that previously, the code took and put a reference of the netdevice.
Because the mlxsw_sp pointer is now obtained from the notifier block, the
port pointer (non-) NULL-ness is all that's relevant, and the reference
does not need to be taken anymore.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>