rcuperf: Fix perf_type module-parameter description

The rcu_bh rcuperf type was removed by commit 620d2460("rcuperf:
Remove the "rcu_bh" and "sched" torture types"), but it lives on in the
MODULE_PARM_DESC() of perf_type.  This commit therefore changes that
module-parameter description to substitute srcu for rcu_bh.

Signed-off-by: Xiao Yang <ice_yangxiao@163.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>