btrfs-progs: deal with drop_progress properly in fsck
    
    While testing snapshot deletion with dm-log-writes I saw that I was
    failing the fsck sometimes when the fs was actually in the correct
    state.  This is because we only skip blocks on the same level of
    root_item->drop_level.  If the drop_level < the root level then we could
    very well walk into nodes that we wouldn't actually walk into on fs
    mount, because the drop_progress is further ahead in the slot of the
    root.  Instead only process the slots of the nodes that are above the
    drop_progress key.  With this patch in place we no longer improperly
    fail to check fs'es that have a drop_progress set with a drop_level <
    root level.
    
    Signed-off-by: Josef Bacik <josef@toxicpanda.com>
    Signed-off-by: David Sterba <dsterba@suse.com>