BACKPORT: FROMLIST: block: blk-crypto for Inline Encryption

We introduce blk-crypto, which manages programming keyslots for struct
bios. With blk-crypto, filesystems only need to call bio_crypt_set_ctx with
the encryption key, algorithm and data_unit_num; they don't have to worry
about getting a keyslot for each encryption context, as blk-crypto handles
that. Blk-crypto also makes it possible for layered devices like device
mapper to make use of inline encryption hardware.

Blk-crypto delegates crypto operations to inline encryption hardware when
available, and also contains a software fallback to the kernel crypto API.
For more details, refer to Documentation/block/inline-encryption.rst.

Bug: 137270441
Test: tested as series; see Ie1b77f7615d6a7a60fdc9105c7ab2200d17636a8
Change-Id: I7df59fef0c1e90043b1899c5a95973e23afac0c5
Signed-off-by: Satya Tangirala <satyat@google.com>
Link: https://lore.kernel.org/linux-fscrypt/20191028072032.6911-4-satyat@google.com/