ixbge: Protect ixgbe_down with __IXGBE_DOWN bit

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

Signed-off-by: Mark Rustad <mark.d.rustad@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>