If you remove an unused spare and then reinsert it, zfsd will now online
it in all pools.
Do not MFC without 2a58b312b6 (but it's ok to MFC that one without this
one).
Submitted by: Ameer Hamza <ahamza@ixsystems.com> (zfsd), Me (tests)
MFC after: 2 weeks
MFC with: 2a58b312b6
Sponsored by: iX Systems, Axcient
Pull Request: https://github.com/freebsd/freebsd-src/pull/697