btrfs: add test for clone operation

btrfs: add test for clone operation

This is a test to verify that the btrfs ioctl clone operation is
able to clone extents of a file to different positions of the file,
that is, the source and target files are the same. Existing tests
only cover the case where the source and target files are different.

Signed-off-by: Filipe David Borba Manana <fdmanana@gmail.com>
Reviewed-by: David Disseldorp <ddiss@suse.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>