BACKPORT: FROMLIST: block: Add encryption context to struct bio

We must have some way of letting a storage device driver know what
encryption context it should use for en/decrypting a request. However,
it's the filesystem/fscrypt that knows about and manages encryption
contexts. As such, when the filesystem layer submits a bio to the block
layer, and this bio eventually reaches a device driver with support for
inline encryption, the device driver will need to have been told the
encryption context for that bio.

We want to communicate the encryption context from the filesystem layer
to the storage device along with the bio, when the bio is submitted to the
block layer. To do this, we add a struct bio_crypt_ctx to struct bio, which
can represent an encryption context (note that we can't use the bi_private
field in struct bio to do this because that field does not function to pass
information across layers in the storage stack). We also introduce various
functions to manipulate the bio_crypt_ctx and make the bio/request merging
logic aware of the bio_crypt_ctx.

Bug: 137270441
Test: tested as series; see I26aac0ac7845a9064f28bb1421eb2522828a6dec
Change-Id: I16d99bb97f8cd7971cc11281a0d7120c5f87d83c
Signed-off-by: Satya Tangirala <satyat@google.com>
Link: https://patchwork.kernel.org/patch/11214719/