BTRFS_IOC_TRANS_START should be privilegued

As mentioned in the comment next to it btrfs_ioctl_trans_start can
do bad damage to filesystems and thus should be limited to privilegued
users.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index da8de6c..6fb4558 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -706,6 +706,9 @@
 	struct btrfs_trans_handle *trans;
 	int ret = 0;
 
+	if (!capable(CAP_SYS_ADMIN))
+		return -EPERM;
+
 	mutex_lock(&root->fs_info->fs_mutex);
 	if (file->private_data) {
 		ret = -EINPROGRESS;