Manually clear text references on reclaim for nullfs and tmpfs.

Both filesystems do no use vnode_pager_dealloc() which would handle
this case otherwise. Nullfs because vnode vm_object handle never
points to nullfs vnode. Tmpfs because its vm_object is never vnode
object at all.

Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week