Illumos 6681 - zfs list burning lots of time in dodefault() via dsl_prop_*

Illumos 6681 - zfs list burning lots of time in dodefault() via dsl_prop_*

6681 zfs list burning lots of time in dodefault() via dsl_prop_*
Reviewed by: Patrick Mooney <patrick.mooney@joyent.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Dan McDonald <danmcd@omniti.com>
Approved by: Matthew Ahrens <mahrens@delphix.com>

References:
  https://www.illumos.org/issues/6681
  https://github.com/illumos/illumos-gate/commit/d09e447

Ported-by: kernelOfTruth kerneloftruth@gmail.com
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #4406