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
a7cb473513