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/projects/illumos-gate//issues/5138
  https://reviews.csiden.org/r/85/

Porting notes:

Since zfs_free_max_blocks is a uint64_t and param_ops_ullong only
exists on kernels >= 3.17-rc1, look for it (param_ops_ullong) and
only if that exists, export it to the /sys filesystem.

Ported by: Turbo Fredriksson <turbo@bayour.com>