rcu: Update rcu_report_qs_rsp() comment

The header comment for rcu_report_qs_rsp() was obsolete, dating well
before the advent of RCU grace-period kthreads.  This commit therefore
brings this comment back into alignment with current reality.

Reported-by: Lihao Liang <lihao.liang@cs.ox.ac.uk>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>