btrfs: regression test for file range cloning

btrfs: regression test for file range cloning

Test btrfs file range cloning with the same file as a source and
destination.

This tests a specific scenario where the extent layout of the file
confused the clone ioctl implementation making it return -EEXIST to
userspace. This issue was fixed by the following linux kernel patch:

   Btrfs: fix range cloning when same inode used as source and destination

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