smb3.11: replace a 4 with server->vals->header_preamble_size

More cleanup of use of hardcoded 4 byte RFC1001 field size

Signed-off-by: Steve French <smfrench@gmail.com>
Reviewed-by: Pavel Shilovsky <pshilov@microsoft.com>
Reviewed-by: Ronnie Sahlberg <lsahlber@redhat.com>