Illumos 6171 - dsl_prop_unregister() slows down dataset eviction.

Illumos 6171 - dsl_prop_unregister() slows down dataset eviction.

6171 dsl_prop_unregister() slows down dataset eviction.
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Prakash Surya <prakash.surya@delphix.com>
Approved by: Dan McDonald <danmcd@omniti.com>

References:
  https://www.illumos.org/issues/6171
  https://github.com/illumos/illumos-gate/commit/03bad06

Porting notes:
  - Conflicts
    - 3558fd7 Prototype/structure update for Linux
    - 2cf7f52 Linux compat 2.6.39: mount_nodev()
    - 13fe019 Illumos #3464
    - 241b541 Illumos 5959 - clean up per-dataset feature count code
  - dsl_prop_unregister() preserved until out of tree consumers
    like Lustre can transition to dsl_prop_unregister_all().
  - Fixing 'space or tab at end of line' in include/sys/dsl_dataset.h

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