libgfs2: Add function to map block states to names
    
    Add an lgfs2_blkst_str() function mapping block states to names and
    replace the duplicated allocdesc arrays defined in gfs2_edit and
    fsck.gfs2. Implemented as a function instead of an array for safety,
    which allowed some range checks to be removed.
    
    Signed-off-by: Andrew Price <anprice@redhat.com>