libgfs2: Add functions for finding free extents
    
    Port gfs2_rbm_find and some functions which it depends on from the gfs2
    kernel code. This will set the base for allocation of single-extent
    files. The functions have been simplified where possible as libgfs2
    doesn't have a concept of reservations for the time being.
    
    Signed-off-by: Andrew Price <anprice@redhat.com>