xfs_scrub_all: survive systemd restarts when waiting for services

xfs_scrub_all: survive systemd restarts when waiting for services

If xfs_scrub_all detects a running systemd, it will use it to invoke
xfs_scrub subprocesses in a sandboxed and resource-controlled
environment.  Unfortunately, if you happen to restart dbus or systemd
while it's running, you get this:

systemd[1]: Reexecuting.
xfs_scrub_all[9958]: Warning! D-Bus connection terminated.
xfs_scrub_all[9956]: Warning! D-Bus connection terminated.
xfs_scrub_all[9956]: Failed to wait for response: Connection reset by peer
xfs_scrub_all[9958]: Failed to wait for response: Connection reset by peer
xfs_scrub_all[9930]: Scrubbing / done, (err=1)
xfs_scrub_all[9930]: Scrubbing /storage done, (err=1)

The xfs_scrub units themselves are still running, it's just that the
`systemctl start' command that xfs_scrub_all uses to start and wait for
the unit lost its connection to dbus and hence is no longer monitoring
sub-services.

When this happens, we don't have great options -- systemctl doesn't have
a command to wait on an activating (aka running) unit.  Emulate the
functionality we normally get by polling the failed/active statuses.

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