ksmbd: fix wrong signingkey creation when encryption is AES256

ksmbd: fix wrong signingkey creation when encryption is AES256

MacOS and Win11 support AES256 encrytion and it is included in the cipher
array of encryption context. Especially on macOS, The most preferred
cipher is AES256. Connecting to ksmbd fails on newer MacOS clients that
support AES256 encryption. MacOS send disconnect request after receiving
final session setup response from ksmbd. Because final session setup is
signed with signing key was generated incorrectly.
For signging key, 'L' value should be initialized to 128 if key size is
16bytes.

Cc: stable@vger.kernel.org
Reported-by: Miao Lihua <441884205@qq.com>
Tested-by: Miao Lihua <441884205@qq.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>