bonding/xfrm: use real_dev instead of slave_dev

Rather than requiring every hw crypto capable NIC driver to do a check for
slave_dev being set, set real_dev in the xfrm layer and xso init time, and
then override it in the bonding driver as needed. Then NIC drivers can
always use real_dev, and at the same time, we eliminate the use of a
variable name that probably shouldn't have been used in the first place,
particularly given recent current events.

CC: Boris Pismenny <borisp@mellanox.com>
CC: Saeed Mahameed <saeedm@mellanox.com>
CC: Leon Romanovsky <leon@kernel.org>
CC: Jay Vosburgh <j.vosburgh@gmail.com>
CC: Veaceslav Falico <vfalico@gmail.com>
CC: Andy Gospodarek <andy@greyhouse.net>
CC: "David S. Miller" <davem@davemloft.net>
CC: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
CC: Jakub Kicinski <kuba@kernel.org>
CC: Steffen Klassert <steffen.klassert@secunet.com>
CC: Herbert Xu <herbert@gondor.apana.org.au>
CC: netdev@vger.kernel.org
Suggested-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: Jarod Wilson <jarod@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>