Illumos 5047 - don't use atomic_*_nv if you discard the return value

Illumos 5047 - don't use atomic_*_nv if you discard the return value

5047 don't use atomic_*_nv if you discard the return value
Author: Josef 'Jeff' Sipek <josef.sipek@nexenta.com>
Reviewed by: Garrett D'Amore <garrett@damore.org>
Reviewed by: Jason King <jason.brian.king@gmail.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>

References:
  https://www.illumos.org/issues/5047
  https://github.com/illumos/illumos-gate/commit/640c167

Porting Notes:

Several hunks from the original patch where not specific to ZFS
and thus were dropped.

Ported-by: Chris Dunlop <chris@onthe.net.au>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <ryao@gentoo.org>
Issue #3172