doc/RCU/listRCU: Update example function name

listRCU.rst document gives an example with 'ipc_lock()', but the
function has dropped off by commit 82061c57 ("ipc: drop
ipc_lock()").  Because the main logic of 'ipc_lock()' has melded in
'shm_lock()' by the commit, this commit updates the document to use
'shm_lock()' instead.
Reviewed-by: Madhuparna Bhowmik <madhuparnabhowmik04@gmail.com>
Signed-off-by: SeongJae Park <sjpark@amazon.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Documentation/RCU/listRCU.rst

@@ -286,11 +286,11 @@ time the external state changes before Linux becomes aware of the change,
 additional RCU-induced staleness is generally not a problem.
 
 However, there are many examples where stale data cannot be tolerated.
-One example in the Linux kernel is the System V IPC (see the ipc_lock()
-function in ipc/util.c).  This code checks a *deleted* flag under a
+One example in the Linux kernel is the System V IPC (see the shm_lock()
+function in ipc/shm.c).  This code checks a *deleted* flag under a
 per-entry spinlock, and, if the *deleted* flag is set, pretends that the
 entry does not exist.  For this to be helpful, the search function must
-return holding the per-entry lock, as ipc_lock() does in fact do.
+return holding the per-entry spinlock, as shm_lock() does in fact do.
 
 .. _quick_quiz: