This indicates that someone has created a directory called "backup" inside your data directory (/var/lib/pgsql/9.4/data), without postgres having permissions on it.
My guess is at some point you accidentally ran pg_basebackup telling it to store the backup in that directory, and ran it as user root, which created a directory there.
There should be no directory named "backup" inside the data directory, so once you've verified it's not something else that ended up there by mistake, you should just remove it. Basically you should *never* create any files or directories manually inside the data directory.
//Magnus