alpha: Omit needless membar in pmap_reference.

If the pmap is published enough for us to obtain a reference to it
then there's no membar needed.  If it's not then something else is
wrong and we can't use pmap_reference here anyway.  Membars are
needed only on the destruction side to make sure all use, by any
thread, happens-before all freeing in the last user thread.

(riastradh)