Do not assume that we own the use reference on the covered vnode until
we set MNTK_UNMOUNT flag on the mp. Otherwise parallel unmount which
wins race with us could dereference the covered vnode, and we are
left with the locked freed memory.

Reported and tested by: pho
Sponsored by: The FreeBSD Foundation
Approved by: re (gjb)
MFC after: 1 week