The new ->gq_seq grace-period sequence numbers must be shifted down,
which give artifacts when these numbers wrap. This commit therefore
enables rcutorture and rcuperf to handle grace-period sequence numbers
even if they do wrap. It does this by allowing a special subtraction
function to be specified, and this function subtracts before shifting.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
d72193123c