xfs_scrub_all: escape paths being passed to systemd service instances

xfs_scrub_all: escape paths being passed to systemd service instances

systemd doesn't like unit instance names with slashes in them, so it
replaces them with dashes when it invokes the service.  However, it's
not smart enough to convert the dashes to something else, so when it
unescapes the instance name to feed to xfs_scrub, it turns all dashes
into slashes.  "/moo-cow" becomes "-moo-cow" becomes "/moo/cow", which
is wrong.  systemd actually /can/ escape the dashes correctly if it is
told that this is a path (and not a unit name), but it didn't do this
prior to January 2017, so fix this for them.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>