The new test case will test whether btrfs-convert can handle 64G ext*
fs.

This exercise the cctx->total_bytes calculation where multiplying 4K
(unsigned int) and 16777216 (u32) could lead to bit overflow for
unsigned int and got 0, and screw up later free space calculation.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>