Illumos 5138 - add tunable for maximum number of blocks freed in one txg

Reviewed by: Mattew Ahrens mahrens@delphix.com
Reviewed by: Adam Leventhal adam.leventhal@delphix.com
Original author: Max Grossman

References:
  https://www.illumos.org/issues/5138
  https://github.com/illumos/illumos-gate/commit/af3465d

Ported by: Turbo Fredriksson <turbo@bayour.com>