MFC r256956:
Improve ZFS N-way mirror read performance by using load and locality
information.

MFC r260713:
Fix ZFS mirror code for handling multiple DVA's

Also make the addition of the d_rotation_rate binary compatible. This allows
storage drivers compiled for 10.0 to work by preserving the ABI for disks.

Approved by:	re (gjb)
Sponsored by:	Multiplay
Fix previous commit: unbreak build of libkvm by including sys/systm.h
only when _KERNEL is defined.

Approved by:	re@ (implicit)