OpenZFS 7261 - nvlist code should enforce name length limit

OpenZFS 7261 - nvlist code should enforce name length limit

Authored by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Sebastien Roy <sebastien.roy@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Robert Mustacchi <rm@joyent.com>
Approved by: Dan McDonald <danmcd@omniti.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/7261
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/48dd5e6
Closes #6579