Fix undefined behavior in misaligned pointer usage

Detected by UBSan and already fixed upstream.

Cherry-pick:
From aa0218d6a12814fac50b287214f9f3b0b99e11b1 Mon Sep 17 00:00:00 2001
From: Brian Behlendorf <behlendorf1@llnl.gov>
Date: Tue, 7 Jan 2014 23:24:37 +0100
Subject: [PATCH] Fix nvlist 'Bus Error' for Sparc

The mis-aligned memory accesses in nvpair_native_embedded() and
nvpair_native_embedded_array() will cause a 'Bus Error' for
architectures such as Sparc which not fully byte addressible.
To avoid this issue care is taken to avoid dereferencing the
potentially mis-aligned packed nvlist_t.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ned Bass <bass6@llnl.gov>
Signed-off-by: marku89 <mar42@kola.li>
Issue #1700

(kamil)