fsck.gfs2: Fix block size validation
    
    The previous patch had an off-by-one error when looping over
    sdp->sd_heightsize. This fixes that and adds an explicit sanity check
    for sb_bsize. Some new tests are added to cover both valid and invalid
    block size cases.
    
    Resolves: bz#1028388
    
    Signed-off-by: Andrew Price <anprice@redhat.com>