ksmbd: Use HMAC-SHA256 library for message signing and key generation

ksmbd: Use HMAC-SHA256 library for message signing and key generation

Convert ksmbd_sign_smb2_pdu() and generate_key() to use the HMAC-SHA256
library instead of a "hmac(sha256)" crypto_shash.  This is simpler and
faster.  With the library there's no need to allocate memory, no need to
handle errors, and the HMAC-SHA256 code is accessed directly without
inefficient indirect calls and other unnecessary API overhead.

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