e2scrub_all: don't even reap if the config file doesn't allow it

e2scrub_all: don't even reap if the config file doesn't allow it

Dave Chinner complains that the automated on-boot e2scrub reaping takes
a long time (because the lvs command can take a while to run) even
though the automated e2scrub is disabled via e2scrub.conf on his
systems.

We still need the reaping service to kill off stale e2scrub snapshots
after a crash, but it's unnecessary to annoy everyone with slow bootup.
Because we can look for the e2scrub snapshots in /dev/mapper, let's
skip reaping if periodic e2scrub is disabled unless we find evidence of
e2scrub snapshots in /dev.

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