Illumos 5176 - lock contention on godfather zio

5176 lock contention on godfather zio
Reviewed by: Alex Reece alex.reece@delphix.com
Reviewed by: George Wilson george.wilson@delphix.com
Reviewed by: Christopher Siden christopher.siden@delphix.com
Reviewed by: Adam Leventhal ahl@delphix.com

References:
  https://www.illumos.org/issues/5176
  https://reviews.csiden.org/r/103/

Ported by: Turbo Fredriksson <turbo@bayour.com>