Illumos 5310 - Remove always true tests for non-NULL ds->ds_phys

5310 Remove always true tests for non-NULL ds->ds_phys
Author: Justin T. Gibbs <justing@spectralogic.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Will Andrews <willa@spectralogic.com>
Reviewed by: Andriy Gapon <avg@FreeBSD.org>
Approved by: Dan McDonald <danmcd@omniti.com>

References:

https://www.illumos.org/issues/5310
https://github.com/illumos/illumos-gate/commit/d808a4f

Ported-by: Chris Dunlop <chris@onthe.net.au>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>