OpenZFS 6447 - handful of nvpair cleanups

OpenZFS 6447 - handful of nvpair cleanups

Authored by: Steve Dougherty <sdougherty@barracuda.com>
Reviewed by: Josef 'Jeff' Sipek <jeffpc@josefsipek.net>
Reviewed by: Andy Stormont <astormont@racktopsystems.com>
Reviewed by: Garrett D'Amore <garrett@damore.org>
Approved by: Robert Mustacchi <rm@joyent.com>
Reviewed-by: Don Brady <dev.fs.zfs@gmail.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: Giuseppe Di Natale <dinatale2@llnl.gov>
OpenZFS-issue: https://www.illumos.org/issues/6447
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/759e89b
Closes #6581