libgfs2: Move unused gfs2_bmap into fsck.gfs2
    
    struct gfs2_bmap is only ever used in fsck.gfs2 and functions which use
    it were moved out of libgfs2 long ago. Complete that migration by moving
    the definition of struct gfs2_bmap into fsck/fsck.h
    
    Signed-off-by: Andrew Price <anprice@redhat.com>