ksmbd: Remove duplicated codes

ksmbd_neg_token_init_mech_token() and ksmbd_neg_token_targ_resp_token()
share same implementation, unify them.

Signed-off-by: Dawei Li <set_pte_at@outlook.com>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>