xfs_scrub_all: convert systemctl calls to dbus

xfs_scrub_all: convert systemctl calls to dbus

Convert the systemctl invocations to direct dbus calls, which decouples
us from the CLI in favor of direct API calls.  This spares us from some
of the insanity of divining service state from program outputs.

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