ksmbd: use SHA-256 library API instead of crypto_shash API

ksmbd_gen_sd_hash() does not support any other algorithm, so the
crypto_shash abstraction provides no value.  Just use the SHA-256
library API instead, which is much simpler and easier to use.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>