Implement GEOM::rotation_rate for gmirror

If all of the mirror's children have the same rotation rate, report
that. But if they have mixed rotation rates, or if any child has an
unknown rotation rate, report "Unknown".

Sponsored by: Axcient
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D39458

(cherry picked from commit 9309a460b23a8cda8d47c5f775da7fc6472d9925)