linux-user: Add support for a group of btrfs ioctls used for snapshots

linux-user: Add support for a group of btrfs ioctls used for snapshots

This patch implements functionality for following ioctls:

BTRFS_IOC_SNAP_CREATE - Creating a subvolume snapshot

    Create a snapshot of a btrfs subvolume. The snapshot is created using the
    ioctl's third argument that is a pointer to a 'struct btrfs_ioctl_vol_args'
    (which was mentioned in the previous patch). Before calling this ioctl,
    the fields of the structure should be filled with aproppriate values for
    the file descriptor and path of the subvolume for which the snapshot is to
    be created.

BTRFS_IOC_SNAP_DESTROY - Removing a subvolume snapshot

    Delete a snapshot of a btrfs subvolume. The snapshot is deleted using the
    ioctl's third argument that is a pointer to a 'struct btrfs_ioctl_vol_args'
    (which was mentioned in the previous patch). Before calling this ioctl,
    the fields of the structure should be filled with aproppriate values for
    the file descriptor and path of the subvolume for which the snapshot is to
    be deleted.

Implementation notes:

    Since the thunk type 'struct btrfs_ioctl_vol_args' is defined in the
    previous patch, the implementation for these ioctls was straightforward.

Signed-off-by: Filip Bozuta <Filip.Bozuta@syrmia.com>
Tested-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20200823195014.116226-3-Filip.Bozuta@syrmia.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>