fsck.gfs2: Make pass2 go by directory rbtree for performance
    
    This patch speeds up pass2 of fsck.gfs2 for large file systems.
    Before, pass2 was checking the bitmap for every block in the file
    system. If the file system is 15TB, that means looping 15 trillion
    times. With this patch, it merely traverses the inode tree, since
    it needs that information anyway, and already ignores any blocks
    that don't have it. Traversing an empty 15TB file system goes from
    13.5 minutes to 0.00 seconds for pass2.
    
    Signed-off-by: Bob Peterson <rpeterso@redhat.com>