btrfs: remove unused parameter from readahead_tree_block

The parent_transid parameter has been unused since its introduction in
ca7a79ad8dbe2466 ("Pass down the expected generation number when reading
tree blocks").  In reada_tree_block, it was even wrongly set to leafsize.
Transid check is done in the proper read and readahead ignores errors.

Signed-off-by: David Sterba <dsterba@suse.cz>
diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
index 39021bf..1b7e354 100644
--- a/fs/btrfs/ctree.c
+++ b/fs/btrfs/ctree.c
@@ -2298,7 +2298,7 @@
 		if ((search <= target && target - search <= 65536) ||
 		    (search > target && search - target <= 65536)) {
 			gen = btrfs_node_ptr_generation(node, nr);
-			readahead_tree_block(root, search, blocksize, gen);
+			readahead_tree_block(root, search, blocksize);
 			nread += blocksize;
 		}
 		nscan++;
@@ -2350,9 +2350,9 @@
 	}
 
 	if (block1)
-		readahead_tree_block(root, block1, blocksize, 0);
+		readahead_tree_block(root, block1, blocksize);
 	if (block2)
-		readahead_tree_block(root, block2, blocksize, 0);
+		readahead_tree_block(root, block2, blocksize);
 }