xfs_scrub: automatic downgrades to dry-run mode in service mode

xfs_scrub: automatic downgrades to dry-run mode in service mode

When service mode is enabled, xfs_scrub is being run within the context
of a systemd service.  The service description language doesn't have any
particularly good constructs for adding in a '-n' argument if the
filesystem is readonly, which means that xfs_scrub is passed a path, and
needs to switch to dry-run mode on its own if the fs is mounted
readonly or the kernel doesn't support repairs.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>