Change VERIFY to ASSERT in mutex_destroy()

There have been multiple reports of 'zdb' tripping the VERIFY in
mutex_destroy() because pthread_mutex_destroy() returns EBUSY.

Exactly how this can happen still needs to be explained, but this
doesn't strictly need to be fatal for non-debug builds.  Therefore,
this patch converts the VERIFY to an ASSERT until the root cause
is determined and resolved.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #2027

lib/libzpool/kernel.c

@@ -289,7 +289,7 @@ mutex_destroy(kmutex_t *mp)
 {
 	ASSERT3U(mp->m_magic, ==, MTX_MAGIC);
 	ASSERT3P(mp->m_owner, ==, MTX_INIT);
-	VERIFY3S(pthread_mutex_destroy(&(mp)->m_lock), ==, 0);
+	ASSERT0(pthread_mutex_destroy(&(mp)->m_lock));
 	mp->m_owner = MTX_DEST;
 	mp->m_magic = 0;
 }