btrfs: Deprecate userspace transaction ioctls

Userspace transactions were introduced in commit 6bf13c0cc833 ("Btrfs:
transaction ioctls") to provide semantics that Ceph's object store
required. However, things have changed significantly since then, to the
point where btrfs is no longer suitable as a backend for ceph and in
fact it's actively advised against such usages. Considering this, there
doesn't seem to be a widespread, legit use case of userspace
transaction. They also clutter the file->private pointer.

So to end the agony let's nuke the userspace transaction ioctls. As a
first step let's give time for people to voice their objection by just
WARN()ining when the userspace transaction is used.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ move the warning past perm checks, keep the has-been-printed state;
  we're ok with just one warning over all filesystems ]
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ioctl.c

@@ -3967,11 +3967,22 @@ static long btrfs_ioctl_trans_start(struct file *file)
 	struct btrfs_root *root = BTRFS_I(inode)->root;
 	struct btrfs_trans_handle *trans;
 	int ret;
+	static bool warned = false;
 
 	ret = -EPERM;
 	if (!capable(CAP_SYS_ADMIN))
 		goto out;
 
+	if (!warned) {
+		btrfs_warn(fs_info,
+			"Userspace transaction mechanism is considered "
+			"deprecated and slated to be removed in 4.17. "
+			"If you have a valid use case please "
+			"speak up on the mailing list");
+		WARN_ON(1);
+		warned = true;
+	}
+
 	ret = -EINPROGRESS;
 	if (file->private_data)
 		goto out;