xfrm: allow driver to quietly refuse offload

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>