Normal rcutorture checking overestimates grace periods somewhat due to
the fact that there is a delay from a grace-period request until the
start of the corresponding grace period and another delay from the end
of that grace period to notification of the requestor.  This means that
rcutorture's detection of RCU bugs is less sensitive than it might be.

It turns out that rcutorture also checks the underlying grace-period
"completed" counter (displayed in Reader Batch output), which in theory
allows rcutorture to do exact checks.  In practice, memory misordering
(by both compiler and CPU) can result in false positives.  However,
experience on x86 shows that these false positives are quite rare,
occurring less than one time per 1,000 hours of testing.  This commit
therefore does the exact checking, giving a warning if any Reader Batch
blows happen, and flagging an error if they happen more often than
once every three hours in long tests.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>