BGP: Maintain valid route attribute flags even in local tables

BGP: Maintain valid route attribute flags even in local tables

BGP route attributes have flags (Optional, Transitive) that are validated
on decode and set to valid value on export. But if such attribute is
modified by filter or set internally by BGP during import, then its flags
would be zero in local tables. That usually does not matter, as they are
not used locally and they were fixed on export, but invalid flags leaked
in BMP and MRT dumps.

Keep route attribute flags set to valid values even when set by filters
or modified by BGP.