tmpfs_reclaim: detach unlinked node on dereferencing.

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