Change 'filler' to 'flags' in on-disk 'raw_locn' structure.

Change 'filler' to 'flags' in on-disk 'raw_locn' structure.

Future patches will make use of a specific flag in the on-disk 'raw_locn'
structure to enable/disable metadata areas, and facilitate metadata
balancing.

Note that 'filler' is always set to '0' (see add_mda() - memset),
so use of this area as a non-zero flags field is a safe way to
provide future code features.

Signed-off-by: Dave Wysochanski <dwysocha@redhat.com>