btrfs: Require CAP_SYS_ADMIN for filesystem rebalance

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>

fs/btrfs/volumes.c

@@ -22,6 +22,7 @@
 #include <linux/blkdev.h>
 #include <linux/random.h>
 #include <linux/iocontext.h>
+#include <linux/capability.h>
 #include <asm/div64.h>
 #include "compat.h"
 #include "ctree.h"
@@ -2024,6 +2025,9 @@ int btrfs_balance(struct btrfs_root *dev_root)
 	if (dev_root->fs_info->sb->s_flags & MS_RDONLY)
 		return -EROFS;
 
+	if (!capable(CAP_SYS_ADMIN))
+		return -EPERM;
+
 	mutex_lock(&dev_root->fs_info->volume_mutex);
 	dev_root = dev_root->fs_info->dev_root;