Fix hotspare_onoffline_003_neg from the ZFS test suite

tests/sys/cddl/zfs/tests/hotspare/hotspare_onoffline_003_neg.ksh
tests/sys/cddl/zfs/tests/hotspare/hotspare_test.sh
Fix hotspare_onoffline_003_neg by simplifying it. It originally
tested that "zpool (on|off)line" wouldn't work on spares, but failed
because those commands _do_ work on active spares. AFAICT they
always have on both Illumos and FreeBSD. The documentation isn't
100% clear, but the justification for preventing them from working
on inactive spares seems to be that those devices don't have any I/O
to stop. That justification doesn't make sense for active spares.
So it's reasonable to allow active spares to be offlined or onlined.

Sponsored by: Spectra Logic Corp