MFC r271026,r271027,r271028:

r271026 (by pjd):

Fix descriptors leak in case of nvlist_xunpack() failure.

Submitted by: Mariusz Zaborski <oshogbo@FreeBSD.org>

r271027 (by pjd):

Declare i.

Reported by: sbruno

r271028 (by pjd):

Use better type for i.