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.auAcked-by: Jonathan Corbet <corbet@lwn.net>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

Documentation/trace/ftrace-uses.rst

@@ -116,6 +116,8 @@ called by a callback may also be traced, and call that same callback,
 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 @@ can help in this regard. If you start your code with:
 
 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 @@ protection, it is best to make sure that RCU is "watching", otherwise
 that data or critical section will not be protected as expected. In this
 case add:
 
+.. code-block:: c
+
 	if (!rcu_is_watching())
 		return;