btrfs: support resizing

[jelly]

Page: storage

Btrfs filesystems should be resizable by Cockpit, btrfs itself only supports online resizing while in Cockpit all our resizing happens offline. We even explicitly check in client.js by querying the fsinfo if offline grow/shrink is supported.

Our resize dialog code also checks if the filesystem is in use:

    const usage = get_active_usage(client,
                                   block && info.grow_needs_unmount ? block.path : null,
                                   _("grow"), null,
                                   true);

And does unmounting if required. For btrfs the Filesystem.BTRFS object has it's own resize method which we need to call instead of Filesystem.Resize().

So the question becomes:

• Do we special case btrfs and allow only online growing/shrinking for that filesystem?
• Do we allow online resizing where supported, i.e. ext4/xfs support online grow (not shrink)?

[jelly]

@KKoukiou We also need to think about this in relation to Anaconda, when partitioning if you want to resize btrfs you'll have to mount, resize, unmount do we really want Cockpit to support that use case?

[jelly]

Cockpit makes the assumption that a filesystem which supports OFFLINE_GROW and OFFLINE_SHRINK may also support ONLINE_GROW/SHRINK. However btrfs only supports ONLINE_GROW and ONLINE_SHRINK so the shrink/grow button is disabled in Cockpit currently.