Filesystem rebalancing (BTRFS_IOC_BALANCE) affects the entire
filesystem and may run uninterruptibly for a long time.  This does not
seem to be something that an unprivileged user should be able to do.
Reported-by: NAron Xu <happyaron.xu@gmail.com>
Signed-off-by: NBen Hutchings <ben@decadent.org.uk>
Signed-off-by: NChris Mason <chris.mason@oracle.com>