btrfs: return void from readahead_tree_block

Errors in readahead are not fatal and ignored elsewhere in the code.

Signed-off-by: David Sterba <dsterba@suse.cz>
diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 058abd0..e0468a9 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -7486,9 +7486,7 @@
 				continue;
 		}
 reada:
-		ret = readahead_tree_block(root, bytenr, blocksize);
-		if (ret)
-			break;
+		readahead_tree_block(root, bytenr, blocksize);
 		nread++;
 	}
 	wc->reada_slot = slot;