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 <[e-mail]>
Signed-off-by: Ben Hutchings <[e-mail]>
Signed-off-by: Chris Mason <[e-mail]>

fs/btrfs/volumes.c

···
 #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"
···
 
 	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;