e1000: Hold RTNL when e1000_down can be called

e1000_down calls netif_queue_set_napi, which assumes that RTNL is held.

There are a few paths for e1000_down to be called in e1000 where RTNL is
not currently being held:
  - e1000_shutdown (pci shutdown)
  - e1000_suspend (power management)
  - e1000_reinit_locked (via e1000_reset_task delayed work)
  - e1000_io_error_detected (via pci error handler)

Hold RTNL in three places to fix this issue:
  - e1000_reset_task: igc, igb, and e100e all hold rtnl in this path.
  - e1000_io_error_detected (pci error handler): e1000e and ixgbe hold
    rtnl in this path. A patch has been posted for igc to do the same
    [1].
  - __e1000_shutdown (which is called from both e1000_shutdown and
    e1000_suspend): igb, ixgbe, and e1000e all hold rtnl in the same
    path.

The other paths which call e1000_down seemingly hold RTNL and are OK:
  - e1000_close (ndo_stop)
  - e1000_change_mtu (ndo_change_mtu)

Based on the above analysis and mailing list discussion [2], I believe
adding rtnl in the three places mentioned above is correct.

Fixes: 8f7ff18a ("e1000: Link NAPI instances to queues and IRQs")
Reported-by: Dmitry Antipov <dmantipov@yandex.ru>
Closes: https://lore.kernel.org/netdev/8cf62307-1965-46a0-a411-ff0080090ff9@yandex.ru/
Link: https://lore.kernel.org/netdev/20241022215246.307821-3-jdamato@fastly.com/ [1]
Link: https://lore.kernel.org/netdev/ZxgVRX7Ne-lTjwiJ@LQ3V64L9R2/

 [2]
Signed-off-by: Joe Damato <jdamato@fastly.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>