MFV r277430: 5313 Allow I/Os to be aggregated across ZIO priority classes
Reviewed by: Andriy Gapon <avg@FreeBSD.org>
Reviewed by: Will Andrews <willa@SpectraLogic.com>
Reviewed by: Matt Ahrens <mahrens@delphix.com>
Reviewed by: George Wilson <george@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Justin T. Gibbs <justing@spectralogic.com>

illumos/illumos-gate@fe319232d24f4ae183730a5a24a09423d8ab4429