ksmbd: remove smb2_buf_length in smb2_transform_hdr

To move smb2_transform_hdr to smbfs_common, This patch remove
smb2_buf_length variable in smb2_transform_hdr.

Cc: Ronnie Sahlberg <ronniesahlberg@gmail.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>