bonding: support hardware encryption offload to slaves

Currently, this support is limited to active-backup mode, as I'm not sure
about the feasilibity of mapping an xfrm_state's offload handle to
multiple hardware devices simultaneously, and we rely on being able to
pass some hints to both the xfrm and NIC driver about whether or not
they're operating on a slave device.

I've tested this atop an Intel x520 device (ixgbe) using libreswan in
transport mode, succesfully achieving ~4.3Gbps throughput with netperf
(more or less identical to throughput on a bare NIC in this system),
as well as successful failover and recovery mid-netperf.

v2: just use CONFIG_XFRM_OFFLOAD for wrapping, isolate more code with it

CC: Jay Vosburgh <[email protected]>
CC: Veaceslav Falico <[email protected]>
CC: Andy Gospodarek <[email protected]>
CC: "David S. Miller" <[email protected]>
CC: Jeff Kirsher <[email protected]>
CC: Jakub Kicinski <[email protected]>
CC: Steffen Klassert <[email protected]>
CC: Herbert Xu <[email protected]>
CC: [email protected]
CC: [email protected]
Signed-off-by: Jarod Wilson <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

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