Fix strdup conflict on other platforms

In the FreeBSD kernel the strdup signature is:

```
char	*strdup(const char *__restrict, struct malloc_type *);
```

It's unfortunate that the developers have chosen to change
the signature of libc functions - but it's what I have to
deal with.

Reviewed-by: Jorgen Lundman <lundman@lundman.net>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #9433

cmd/ztest/ztest.c

@@ -2863,7 +2863,7 @@ ztest_spa_upgrade(ztest_ds_t *zd, uint64_t id)
 	    zpool_prop_to_name(ZPOOL_PROP_VERSION)));
 	spa_close(spa, FTAG);
 
-	strfree(name);
+	kmem_strfree(name);
 	mutex_exit(&ztest_vdev_lock);
 }