gfs2_edit: savemeta and restoremeta improvements
    
    This adds various improvements to savemeta and restoremeta including:
    
    - Made the saved_metablock buffer a flexible array to make better use of
      sizeof() etc.
    - Read/write saved_metablock structures with one read/write call instead
      of one per field (gives a small perf improvement).
    - Factor out the block restore code and split the "find highest block"
      code out of restore_data.
    - Print more, clearer information relating to the metadata file,
      progress made and errors.
    - Made global variables static, removed unused ones.
    - Improved code clarity, separation of concerns.
    
    Signed-off-by: Andrew Price <anprice@redhat.com>