btrfs: use btrfs_next_item() at scrub.c:find_first_extent_item()

There's no reason to open code what btrfs_next_item() does when searching
for extent items at scrub.c:scrub.c:find_first_extent_item(), so remove
the logic to find the next item and use btrfs_next_item() instead, making
the code shorter and less nested code blocks. While at it also fix the
comment to the plural "items" instead of "item" and end it with proper
punctuation.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/scrub.c

@@ -1408,14 +1408,11 @@ static int find_first_extent_item(struct btrfs_root *extent_root,
 		if (ret > 0)
 			break;
 next:
-		path->slots[0]++;
-		if (path->slots[0] >= btrfs_header_nritems(path->nodes[0])) {
-			ret = btrfs_next_leaf(extent_root, path);
-			if (ret) {
-				/* Either no more item or fatal error */
-				btrfs_release_path(path);
-				return ret;
-			}
+		ret = btrfs_next_item(extent_root, path);
+		if (ret) {
+			/* Either no more items or a fatal error. */
+			btrfs_release_path(path);
+			return ret;
 		}
 	}
 	btrfs_release_path(path);