ftrace/documentation: Fix RST C code blocks

Some C code in the ftrace-users.rst document is missing RST C block
annotation, which has to be added.

Link: https://lore.kernel.org/r/20201116173502.392a769c@canb.auug.org.au

Acked-by: Jonathan Corbet <corbet@lwn.net>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
diff --git a/Documentation/trace/ftrace-uses.rst b/Documentation/trace/ftrace-uses.rst
index 5981d56..f7d98ae 100644
--- a/Documentation/trace/ftrace-uses.rst
+++ b/Documentation/trace/ftrace-uses.rst
@@ -116,6 +116,8 @@
 recursion protection must be used. There are two helper functions that
 can help in this regard. If you start your code with:
 
+.. code-block:: c
+
 	int bit;
 
 	bit = ftrace_test_recursion_trylock(ip, parent_ip);
@@ -124,6 +126,8 @@
 
 and end it with:
 
+.. code-block:: c
+
 	ftrace_test_recursion_unlock(bit);
 
 The code in between will be safe to use, even if it ends up calling a
@@ -145,6 +149,8 @@
 that data or critical section will not be protected as expected. In this
 case add:
 
+.. code-block:: c
+
 	if (!rcu_is_watching())
 		return;