RFE: Filesystem scrubbing support

[tbzatek]

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Besides the traditional fsck functionality (aka org.freedesktop.UDisks2.Filesystem.Check()) that often requires the filesystem unmounted or readonly, some filesystems do offer scrubbing or similar online consistency check.

XFS: https://docs.kernel.org/filesystems/xfs-online-fsck-design.html#checking-on-demand btrfs: https://btrfs.readthedocs.io/en/latest/btrfs-scrub.html

API-wise not sure how realistic is to expect general support for foreground operations, might need to be modelled as a background start & status method calls, e.g. like SMART background check.

This should be implemented along with #1062 for async fs error/warning notifications.