xfrm: allow driver to quietly refuse offload

If the "offload" attribute is used to create an IPsec SA
and the .xdo_dev_state_add() fails, the SA creation fails.
However, if the "offload" attribute is used on a device that
doesn't offer it, the attribute is quietly ignored and the SA
is created without an offload.

Along the same line of that second case, it would be good to
have a way for the device to refuse to offload an SA without
failing the whole SA creation.  This patch adds that feature
by allowing the driver to return -EOPNOTSUPP as a signal that
the SA may be fine, it just can't be offloaded.

This allows the user a little more flexibility in requesting
offloads and not needing to know every detail at all times about
each specific NIC when trying to create SAs.
Signed-off-by: Shannon Nelson <shannon.nelson@oracle.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

Documentation/networking/xfrm_device.txt

@@ -68,6 +68,10 @@ and an indication of whether it is for Rx or Tx.  The driver should
 	- verify the algorithm is supported for offloads
 	- store the SA information (key, salt, target-ip, protocol, etc)
 	- enable the HW offload of the SA
+	- return status value:
+		0             success
+		-EOPNETSUPP   offload not supported, try SW IPsec
+		other         fail the request
 
 The driver can also set an offload_handle in the SA, an opaque void pointer
 that can be used to convey context into the fast-path offload requests.

net/xfrm/xfrm_device.c

@@ -192,8 +192,12 @@ int xfrm_dev_state_add(struct net *net, struct xfrm_state *x,
 
 	err = dev->xfrmdev_ops->xdo_dev_state_add(x);
 	if (err) {
+		xso->num_exthdrs = 0;
+		xso->flags = 0;
 		xso->dev = NULL;
 		dev_put(dev);
+
+		if (err != -EOPNOTSUPP)
 			return err;
 	}