e2scrub: fix systemd escaping again

e2scrub: 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:

 # e2scrub_all
 Scrubbing /home...
 Invalid unit name "e2scrub@/home" was escaped as "e2scrub@-home"
 (maybe you should use systemd-escape?)
 Starting Online ext4 Metadata Check for /home...

So change the escape_path_for_systemd function to escape paths
unconditionally to make the warning go away.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>