The safety of SRCU is provided byy wait_idx() rather than flipping.
The flipping actually prevents starvation.

This commit therefore updates the comments to more accurately and
precisely describe what is going on.
Signed-off-by: NLai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>