rcu: Standardize explicit CPU-hotplug calls

rcu_report_dead() and rcutree_migrate_callbacks() have their headers in
rcupdate.h while those are pure rcutree calls, like the other CPU-hotplug
functions.

Also rcu_cpu_starting() and rcu_report_dead() have different naming
conventions while they mirror each other's effects.

Fix the headers and propose a naming that relates both functions and
aligns with the prefix of other rcutree CPU-hotplug functions.

Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>

Documentation/RCU/Design/Memory-Ordering/TreeRCU-hotplug.svg

@@ -607,7 +607,7 @@
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        font-size="192"
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-       style="font-size:192px;font-style:normal;font-weight:bold;text-anchor:start;fill:#000000;stroke-width:0.025in;font-family:Courier">rcu_report_dead()</text>
+       style="font-size:192px;font-style:normal;font-weight:bold;text-anchor:start;fill:#000000;stroke-width:0.025in;font-family:Courier">rcutree_report_cpu_dead()</text>
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        xml:space="preserve"
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@@ -728,7 +728,7 @@
        font-style="normal"
        y="3679.27"
        x="-3804.9949"
-       xml:space="preserve">rcu_cpu_starting()</text>
+       xml:space="preserve">rcutree_report_cpu_starting()</text>
     <g
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        id="g3107-7-5-0"