ftrace: Fix missing synchronize_rcu() removing trampoline from kallsyms

Add synchronize_rcu() after list_del_rcu() in
ftrace_remove_trampoline_from_kallsyms() to protect readers of
ftrace_ops_trampoline_list (in ftrace_get_trampoline_kallsym)
which is used when kallsyms is read.

Link: https://lkml.kernel.org/r/20200901091617.31837-1-adrian.hunter@intel.com

Fixes: fc0ea79 ("ftrace: Add symbols for ftrace trampolines")
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/ftrace.c

@@ -2782,6 +2782,7 @@ static void ftrace_remove_trampoline_from_kallsyms(struct ftrace_ops *ops)
 {
 	lockdep_assert_held(&ftrace_lock);
 	list_del_rcu(&ops->list);
+	synchronize_rcu();
 }
 
 /*