btrfs: fix unprivileged snapshot creation
btrfs: fix unprivileged snapshot creation

We already fixed privileged btrfs snapshot creation in:

commit 1c7222c084769a1d9406ca7dab943d8a5f016a56
Author: Christian Brauner <48455ab3070520a2d174545c7239d6d0fabd9a83@ubuntu.com>
Date:   Tue Nov 28 13:51:03 2017 +0100

    btrfs: fix btrfs_snapshot()

    Closes #1956.

    Signed-off-by: Christian Brauner <48455ab3070520a2d174545c7239d6d0fabd9a83@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: 899387868b07db3f59404ef0c28fa2107a63f859@gmail.com
Signed-off-by: Christian Brauner <48455ab3070520a2d174545c7239d6d0fabd9a83@ubuntu.com>

caglar10ur/lxc,hallyn/lxc,caglar10ur/lxc,ss1h2a3tw/lxc,ss1h2a3tw/lxc,thmo/lxc,thmo/lxc,hallyn/lxc,caglar10ur/lxc,hallyn/lxc,ss1h2a3tw/lxc,thmo/lxc