Illumos #3006

3006 VERIFY[S,U,P] and ASSERT[S,U,P] frequently check if first argument is zero
Reviewed by Matt Ahrens <matthew.ahrens@delphix.com>
Reviewed by George Wilson <george.wilson@delphix.com>
Approved by Eric Schrock <eric.schrock@delphix.com>

References:
  illumos/illumos-gate@fb09f5aad449c97fe309678f3f604982b563a96f
  https://www.illumos.org/issues/3006

This is a fairly cosmetic change for ZoL, however, it will help in merging
subsequent upstream commits.

For ZoL, this issue requres a commit to both the spl and the zfs
repositories.

Ported-by: Tim Chase <tim@chase2k.com>