Otherwise it is dereferenced one extra time at unmount, if it survives
long enough. One way to hold the reference on such node is to keep it
open.
tmpfs_vptocnp() now needs to account for the possibility that unlocked
node was removed from the list.
Reported by: danfe
Tested by: danfe, pho
MFC after: 1 week
Sponsored by: The FreeBSD Foundation