MFC r337219: MFV r337218: 7261 nvlist code should enforce name length limit

illumos/illumos-gate@48dd5e630c9b1773b7b10d08a3b90b6c9062d713

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>
Author: Matthew Ahrens <mahrens@delphix.com>