Fix zfsd with the device_removal pool feature.

[asomers]

Summary

Previously zfsd would crash in the presence of a pool with a
top-level-vdev that had previously been removed. The crash only happens
in stable/14 and later, probably do to differences in libcxx, but the
change should be MFCed anyway.

PR: 273663
Reported by: Marek Zarychta <[e-mail]>
MFC after: 1 week
Sponsored by: Axcient

[mav]

cddl/usr.sbin/zfsd/vdev_iterator.cc

112

I personally would initialize vdevConfig above in declaration, but whatever.