xfs_scrub: use dynamic users when running as a systemd service

xfs_scrub: use dynamic users when running as a systemd service

Five years ago, systemd introduced the DynamicUser directive that
allocates a new unique user/group id, runs a service with those ids, and
deletes them after the service exits.  This is a good replacement for
User=nobody, since it eliminates the threat of nobody-services messing
with each other.

Make this transition ahead of all the other security tightenings that
will land in the next few patches, and add credits for the people who
suggested the change and reviewed it.

Link: https://0pointer.net/blog/dynamic-users-with-systemd.html
Suggested-by: Helle Vaanzinn <glitsj16@riseup.net>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>