ixgbevf: Protect ixgbevf_down with __IXGBEVF_DOWN bit

The ixgbevf_down function can now prevent multiple executions by
doing test_and_set_bit on __IXGBEVF_DOWN. This did not work before
introduction of the __IXGBEVF_REMOVING bit, because of overloading
of __IXGBEVF_DOWN. Also add smp_mb__before_clear_bit call before
clearing the __IXGBEVF_DOWN bit.

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>