btrfs: scrub: use find_first_extent_item to for extent item search

Since we have find_first_extent_item() to iterate the extent items of a
certain range, there is no need to use the open-coded version.

Replace the final scrub call site with find_first_extent_item().

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