OPTIM: ring: avoid reloading the tail_ofs value before the CAS in ring_write()

OPTIM: ring: avoid reloading the tail_ofs value before the CAS in ring_write()

The load followed by the CAS seem to cause two bus cycles, one to
retrieve the cache line in shared state and a second one to get
exclusive ownership of it. Tests show that on x86 it's much better
to just rely on the previous value and preset it to zero before
entering the loop. We just mask the ring lock in case of failure
so as to challenge it on next iteration and that's done.

This little change brings 2.3% extra performance (11.34M msg/s) on
a 64-core AMD.