lib/crypto: aes: Add XTS support

Add support for AES-XTS to the crypto library.

This will be used to provide a streamlined implementation of the
"xts(aes)" crypto_skcipher algorithm.  I'm also planning to use this
directly in fscrypt and blk-crypto-fallback.

As usual, the architecture-optimized AES-XTS code will be migrated into
the library as well (using the hooks provided in this commit),
eliminating lots of repetitive boilerplate code.  Compared to direct
implementation of "xts(aes)", I've also eliminated the requirement for
architectures to implement ciphertext stealing, as the library just
handles it portably instead.  That will simplify things considerably.

Initial test coverage is provided by the crypto_skcipher support added
in a later commit.  I'm planning a KUnit test suite as well.

Link: https://patch.msgid.link/20260715221153.246410-6-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>

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