xfrm: Fix NETDEV_DOWN with IPSec offload

Upon NETDEV_DOWN event, all xfrm_state objects which are bound to
the device are flushed.

The condition for this is wrong, though, testing dev->hw_features
instead of dev->features. If a device has non-user-modifiable
NETIF_F_HW_ESP, then its xfrm_state objects are not flushed,
causing a crash later on after the device is deleted.

Check dev->features instead of dev->hw_features.

Fixes: d77e38e6 ("xfrm: Add an IPsec hardware offloading API")
Signed-off-by: Ilan Tayari <ilant@mellanox.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>