ixgbe: fix race condition with shutdown

It was possible for shutdown to pull the rug out from other driver entry
points.  Now we just grab the rtnl lock before taking everything apart.
Thanks to Hariharan for noticing this tight race condition.

Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com>
Cc: Hariharan Nagarajan <hanagara@cisco.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>