UBUNTU: SAUCE: shiftfs: allow changing ro/rw for subvolumes

UBUNTU: SAUCE: shiftfs: allow changing ro/rw for subvolumes

BugLink: https://bugs.launchpad.net/bugs/1832316
This enables toggling between ro/rw for btrfs subvolumes under shiftfs.

Currently, btrfs workloads employing shiftfs cause regression.
With btrfs unprivileged users can already toggle whether a subvolume
will be ro or rw. This is broken on current shiftfs as we haven't
whitelisted these ioctls().
To prevent such regression, we need to whitelist the ioctls
BTRFS_IOC_FS_INFO, BTRFS_IOC_SUBVOL_GETFLAGS, and
BTRFS_IOC_SUBVOL_SETFLAGS. All of them should be safe for unprivileged
users.

Cc: Seth Forshee <seth.forshee@canonical.com>
Cc: Tyler Hicks <tyhicks@canonical.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Acked-by: Seth Forshee <seth.forshee@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>