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

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

Porting notes:

Look for it (param_ops_ullong) and depending on that use uint64_t/ullong
for kernels >= v3.17-rc1 where it was introduced and ulong/ulong otherwise
for the zfs_free_max_blocks variable.

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