ring_spsc: revert the previous commit to push()

It's ok to load _begin with relaxed ordering (in the size()) since
store to _ring[end & MaxSizeMask] may not be reordered with it due to
control dependency (see Documentation/memory-barriers.txt in the
Linux tree).

Signed-off-by: Vlad Zolotarov <vladz@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>