zfs receive: -u can be ignored sometimes

When force-receiving a filesystem that was already mounted the re-created
filesystem is mounted despite -u flag.

Also see https://www.illumos.org/issues/6412.

PR: 204705
Tested by: Vladimir Krstulja <vlad-fbsd@acheronmedia.com>
MFC after: 2 weeks
X-Needs-Upstreaming: illumos