zfsd: Don't spare a vdev that's being replaced

If a zfs pool contains a replacing vdev (either created manually by "zpool
replace" or by zfsd(8) via autoreplace by physical path) and then new spares
get added to the pool, zfsd shouldn't use one to replace the drive that is
already being replaced. That's a waste of resources that just slows down
the rebuild.

PR: 225547
MFC after: 3 weeks
Sponsored by: Spectra Logic Corp