Illumos 6450 - scrub/resilver unnecessarily traverses snapshots

6450 scrub/resilver unnecessarily traverses snapshots created
after the scrub started
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Prakash Surya <prakash.surya@delphix.com>
Reviewed by: Richard Elling <Richard.Elling@RichardElling.com>
Approved by: Richard Lowe <richlowe@richlowe.net>

References:

https://www.illumos.org/issues/6450
https://github.com/illumos/illumos-gate/commit/38d6103

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