Right now, dounmount() is called with unreferenced mount point.
Nothing stops a parallel unmount to suceed before the given call to
dounmount() checks and locks the covered vnode. Prevent dounmount()
from acting on the freed (although type-stable) memory by changing the
interface to require the mount point to be referenced. dounmount()
consumes the reference on return, regardless of the sucessfull or
erronous result.

Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks