Illumos 4929 - want prevsnap property

Illumos 4929 - want prevsnap property

4929 want prevsnap property
Reviewed by: Adam Leventhal <adam.leventhal@delphix.com>
Reviewed by: Matt Amdur <matt.amdur@delphix.com>
Reviewed by: Saso Kiselkov <skiselkov.ml@gmail.com>
Reviewed by: Boris Protopopov <bprotopopov@hotmail.com>
Reviewed by: Richard Lowe <richlowe@richlowe.net>
Approved by: Dan McDonald <danmcd@omniti.com>

References:
  https://www.illumos.org/issues/4929
  https://github.com/illumos/illumos-gate/commit/b461c74

Porting notes:
- [include/sys/fs/zfs.h]
  - f67d70 Create an 'overlay' property
  - 11b9ec Add full SELinux support
- [fs/zfs/dsl_dataset.c]
  - This increases the stack size of dsl_dataset_stats() but
    nothing has been changed until this is shown to be an issue.

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