We already fixed privileged btrfs snapshot creation in:

commit 1c7222c0
Author: Christian Brauner <christian.brauner@ubuntu.com>
Date:   Tue Nov 28 13:51:03 2017 +0100

    btrfs: fix btrfs_snapshot()

    Closes #1956.
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Adrian Reber <areber@redhat.com>

but missed unprivileged btrfs snapshot creation. Fix it too.

Follow-up to #1956.
Closes #2051.

Reported-by: Oleg Freedhom overlayfs@gmail.com
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>