xfrm: Add explicit dev to .xdo_dev_state_{add,delete,free}

Previously, device driver IPSec offload implementations would fall into
two categories:
1. Those that used xso.dev to determine the offload device.
2. Those that used xso.real_dev to determine the offload device.

The first category didn't work with bonding while the second did.
In a non-bonding setup the two pointers are the same.

This commit adds explicit pointers for the offload netdevice to
.xdo_dev_state_add() / .xdo_dev_state_delete() / .xdo_dev_state_free()
which eliminates the confusion and allows drivers from the first
category to work with bonding.

xso.real_dev now becomes a private pointer managed by the bonding
driver.

Signed-off-by: Cosmin Ratiu <cratiu@nvidia.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>