Fix an assert to tolerate spare parents with more than 2 children.

This can occur if a spare is being spared, which would yield three
children: the original pool drive, the previous spare, and the spare
that is replacing it.

MFC after: 1 week
Sponsored by: Spectra Logic
Affects: All ZFS versions starting 7 Jun 2006 (illumos 94de1d4c)
MFSpectraBSD: r668345 on 2013/06/04 17:10:43