xfs_scrub_all: fix systemd escaping again

Apparently newer versions of systemd than the one on this author's
laptop <cough> now complain about lack of (path) escaping in unit instance
variable contents:

 # xfs_scrub_all
 Scrubbing /home...
  Invalid unit name "xfs_scrub@/home" was escaped as "xfs_scrub@-home"
 (maybe you should use systemd-escape?)
 Starting Online XFS Metadata Check for /home...

So change the systemd_escape() function to escape paths unconditionally
to make the warning go away.

Reported-by: Matthias Bodenbinder <matthias@bodenbinder.de>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>