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 <[e-mail]>
Cc: Jiri Slaby <[e-mail]>
Cc: [e-mail]
Cc: "Paul E. McKenney" <[e-mail]>
Cc: Josh Triplett <[e-mail]>
Cc: [e-mail]
Reviewed-by: Paul E. McKenney <[e-mail]>
Signed-off-by: Emil Velikov <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

kernel/rcu/tree_stall.h

···
 	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",