rcutorture: Also use GP sequence to judge forward progress

Currently, rcutorture relies solely on the progress of
rcu_torture_writer() to judge grace-period forward progress.  In theory,
this is the gold standard of forward progress, but in practice rcutorture
separately detects and reports rcu_torture_writer() stalls.  This commit
therefore adds the grace-period sequence number (when provided) to the
judgment of grace-period forward progress, which makes it easier to
distinguish between failure of actual grace periods to progress on the
one hand and downstream forward-progress failures on the other.

For example, given this change, if rcu_torture_writer() stalls,
but rcu_torture_fwd_prog() does not complain, then the grace-period
computation is working, which is a hint that the failure lies in callback
processing, wakeup of the rcu_torture_writer() kthread, or similar.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>