As others have pointed out already, DBFS is (even if it should be supported) a bad idea for RMAN backups.
On Exadata, if you want to use the space of the DBFS_DG diskgroup for RMAN backups - just backup directly into it.
Something like backup database format '+DBFS_DG';
There is no need to use the DBFS filesystem for that.
"Don't believe it, test it!"