Correct fences for sys/refcount.h.

The acq barrier in refcount_acquire() has no use, constructor must
ensure that the changes are visible before publication by other means.
Last release must sync/with the constructor and all updaters.

This is based on the refcount/shared_ptr analysis I heard at the Hans
Boehm and Herb Sutter talks about C++ atomics.

Reviewed by: alc, jhb, markj
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D11270