Actually run reap_children during online_offline's cleanup

tests/sys/cddl/zfs/tests/online_offline/cleanup.ksh
tests/sys/cddl/zfs/tests/online_offline/online_offline_001_pos.ksh
tests/sys/cddl/zfs/tests/online_offline/online_offline_002_neg.ksh
It seems that reap_children was intended to run during cleanup, but
the onexit hook was never set. So I just moved it to the
cleanup.ksh script, because all test cases in this directory need
it.

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