rcu: constify sysrq_key_op

With earlier commits, the API no longer discards the const-ness of the
sysrq_key_op. As such we can add the notation.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jslaby@suse.com>
Cc: linux-kernel@vger.kernel.org
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: rcu@vger.kernel.org
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://lore.kernel.org/r/20200513214351.2138580-11-emil.l.velikov@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

kernel/rcu/tree_stall.h

@@ -729,7 +729,7 @@ static void sysrq_show_rcu(int key)
 	show_rcu_gp_kthreads();
 }
 
-static struct sysrq_key_op sysrq_rcudump_op = {
+static const struct sysrq_key_op sysrq_rcudump_op = {
 	.handler = sysrq_show_rcu,
 	.help_msg = "show-rcu(y)",
 	.action_msg = "Show RCU tree",