Currently, you get back "can only attach to mirrors and top-level disks"
unconditionally if zpool attach returns ENOTSUP, but that also happens
if, say, feature@device_rebuild=disabled and you tried attach -s.
So let's print an error for that case, lest people go down a rabbit hole
looking into what they did wrong.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes#11414Closes#12680
1139e170d4