btrfs: Require CAP_SYS_ADMIN for filesystem rebalance

btrfs: Require CAP_SYS_ADMIN for filesystem rebalance

commit 6f88a4403def422bd8e276ddf6863d6ac71435d2 upstream.

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: Aron Xu <happyaron.xu@gmail.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Chris Mason <chris.mason@oracle.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>