Btrfs: use right extent item position in send when finding extent clones

This was a leftover from the commit:

   74dd17fb
   (Btrfs: fix btrfs send for inline items and compression)

Signed-off-by: Filipe David Borba Manana <fdmanana@gmail.com>
Signed-off-by: Josef Bacik <jbacik@fb.com>