repart: support "nodatacow" in btrfs subvolumes

repart: support "nodatacow" in btrfs subvolumes

In btrfs-progs 6.15 it is planned to add a new parameter in mkfs.btrfs
--inode-flags, that can set attributes for subvolumes, directories, and
files.

The current supported attributes are "nodatacow", to disable CoW, and
"nodatasum", to disable the checksum.

This commit extend the "Subvolunes=" option to understand the
"nodatacow" flag for subvolums only.

If RepartOffline is enabled it will build the image without loopback
devices, using the correct --inode-flags parameters.

If RepartOffline is disabled it will use loopback devices and set the
btrfs attributes accordingly.

Signed-off-by: Alberto Planas <aplanas@suse.com>