libsa: Fix a bug in nvlist creation

When adding an entry to an nvlist, the data buffer might need to be
resized. When this happens, the XDR encoder's notion of the buffer size
also needs to be updated, otherwise the operation may erroneously fail.

Reviewed by: tsoome, imp
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35254