btrfs-progs: tests: check that convert does not create extents beyond device boundary
    
    Add a test case to check if the converted fs has device extent beyond
    boundary.
    
    The disk layout of source ext4 fs needs some extents to make them
    allocated at the very end of the fs.  The script is from the original
    reporter.
    
    Also, since the existing convert tests always uses 512M as device size,
    which is not suitable for this test case, make it to grab the existing
    device size to co-operate with this test case.
    
    Reported-by: Jiachen YANG <farseerfc@gmail.com>
    Signed-off-by: Qu Wenruo <wqu@suse.com>
    Signed-off-by: David Sterba <dsterba@suse.com>