ixgbe: Fix rcu warnings induced by LER

Resolve some rcu warnings produced when LER actions take place.
This appears to be due to not holding the rtnl lock when calling
ixgbe_down, so hold the lock. Also avoid disabling the device
when it is already disabled. This check is necessary because the
callback can be called more than once in some cases.

Signed-off-by: Mark Rustad <mark.d.rustad@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>