block/keyslot-manager: Introduce passthrough keyslot manager

The device mapper may map over devices that have inline encryption
capabilities, and to make use of those capabilities, the DM device must
itself advertise those inline encryption capabilities. One way to do this
would be to have the DM device set up a keyslot manager with a
"sufficiently large" number of keyslots, but that would use a lot of
memory. Also, the DM device itself has no "keyslots", and it doesn't make
much sense to talk about "programming a key into a DM device's keyslot
manager", so all that extra memory used to represent those keyslots is just
wasted. All a DM device really needs to be able to do is advertise the
crypto capabilities of the underlying devices in a coherent manner and
expose a way to evict keys from the underlying devices.

There are also devices with inline encryption hardware that do not
have a limited number of keyslots. One can send a raw encryption key along
with a bio to these devices (as opposed to typical inline encryption
hardware that require users to first program a raw encryption key into a
keyslot, and send the index of that keyslot along with the bio). These
devices also only need the same things from the keyslot manager that DM
devices need - a way to advertise crypto capabilities and potentially a way
to expose a function to evict keys from hardware.

So we introduce a "passthrough" keyslot manager that provides a way to
represent a keyslot manager that doesn't have just a limited number of
keyslots, and for which do not require keys to be programmed into keyslots.
DM devices can set up a passthrough keyslot manager in their request
queues, and advertise appropriate crypto capabilities based on those of the
underlying devices. Blk-crypto does not attempt to program keys into any
keyslots in the passthrough keyslot manager. Instead, if/when the bio is
resubmitted to the underlying device, blk-crypto will try to program the
key into the underlying device's keyslot manager.

Signed-off-by: Satya Tangirala <satyat@google.com>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Acked-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>