ANDROID: block: add KSM op to derive software secret from wrapped key

Some inline encryption hardware supports protecting the keys in hardware
and only exposing wrapped keys to software.  To use this capability,
userspace must provide a hardware-wrapped key rather than a raw key.

However, users of inline encryption in the kernel won't necessarily use
the user-specified key directly for inline encryption.  E.g. with
fscrypt with IV_INO_LBLK_64 policies, each user-provided key is used to
derive a file contents encryption key, filenames encryption key, and key
identifier.  Since inline encryption can only be used with file
contents, if the user were to provide a wrapped key there would
(naively) be no way to encrypt filenames or derive the key identifier.

This problem is solved by designing the hardware to internally use the
unwrapped key as input to a KDF from which multiple cryptographically
isolated keys can be derived, including both the inline crypto key (not
exposed to software) and a secret that *is* exposed to software.

Add a function to the keyslot manager to allow upper layers to request
this software secret from a hardware-wrapped key.

Bug: 147209885

Change-Id: Iffb05b297b7ba3f3e865e798e4bb73aef4e6ba19
Co-developed-by: Gaurav Kashyap <gaurkash@codeaurora.org>
Signed-off-by: Gaurav Kashyap <gaurkash@codeaurora.org>
Signed-off-by: Barani Muthukumaran <bmuthuku@codeaurora.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>