urcu-mb/signal/membarrier: improve 2-phase wait scheme

urcu-mb/signal/membarrier: improve 2-phase wait scheme

In the single-bit, 2-phase grace period scheme, all we need to do is to
observe each reader going through a quiescent state while we are in the
grace period.

We therefore only need to perform one global counter update, surrounded
by 2 iterations on readers to observe change in their snapshot.

We can therefore remove the first counter update (prior to the first
iteration on readers): it was useless and was only slowing down the
grace period.

CC: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
CC: Lai Jiangshan <laijs@cn.fujitsu.com>
CC: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>