srcu: Fix rcutorture-statistics typo

The function srcutorture_get_gp_data() duplicated the check for
sp->batch_check0.head instead of also checking sp->batch_check1.head.
The only effect of this typo would be for rcutorture statistics to
understate the fraction of time that an SRCU grace period was in flight,
and only for Classic SRCU.  This commit fixes this typo.

Reported-by: David Binderman <dcb314@hotmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>