btrfs: fix unprivileged snapshot creation

btrfs: fix unprivileged snapshot creation

We already fixed privileged btrfs snapshot creation in:

commit 1c7222c084769a1d9406ca7dab943d8a5f016a56
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>