btrfs: test cloning of zero length ranges

btrfs: test cloning of zero length ranges

Test cloning a file range with a length of zero into a destination offset
greater than zero.

This made btrfs create an extent state record with a start offset greater than
the end offset, resulting in chaos such as an infinite loop when evicting an
inode.

This issue was fixed by the following linux kernel patch:

   Btrfs: fix inode eviction infinite loop after cloning into it

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Dave Chinner <david@fromorbit.com>