fscrypt: Add SM4 XTS/CTS symmetric algorithm support

fscrypt: Add SM4 XTS/CTS symmetric algorithm support

Add support for XTS and CTS mode variant of SM4 algorithm. The former is
used to encrypt file contents, while the latter (SM4-CTS-CBC) is used to
encrypt filenames.

SM4 is a symmetric algorithm widely used in China, and is even mandatory
algorithm in some special scenarios. We need to provide these users with
the ability to encrypt files or disks using SM4-XTS.

Signed-off-by: Tianjia Zhang <[email protected]>
Signed-off-by: Eric Biggers <[email protected]>
Link: https://lore.kernel.org/r/[email protected]