xfs_scrub_all: only use the xfs_scrub@ systemd services in service mode

xfs_scrub_all: only use the xfs_scrub@ systemd services in service mode

Since the per-mount xfs_scrub@.service definition includes a bunch of
resource usage constraints, we no longer want to use those services if
xfs_scrub_all is being run directly by the sysadmin (aka not in service
mode) on the presumption that sysadmins want answers as quickly as
possible.

Therefore, only try to call the systemd service from xfs_scrub_all if
SERVICE_MODE is set in the environment.  If reaching out to systemd
fails and we're in service mode, we still want to run xfs_scrub
directly.  Split the makefile variables as necessary so that we only
pass -b to xfs_scrub in service mode.

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