mm, kcsan: Enable barrier instrumentation

Some memory management calls imply memory barriers that are required to
avoid false positives. For example, without the correct instrumentation,
we could observe data races of the following variant:

                   T0           |           T1
        ------------------------+------------------------
                                |
         *a = 42;    ---+       |
         kfree(a);      |       |
                        |       | b = kmalloc(..); // b == a
          <reordered> <-+       | *b = 42;         // not a data race!
                                |

Therefore, instrument memory barriers in all allocator code currently
not being instrumented in a default build.
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

mm/Makefile

@@ -15,6 +15,8 @@ KCSAN_SANITIZE_slab_common.o := n
 KCSAN_SANITIZE_slab.o := n
 KCSAN_SANITIZE_slub.o := n
 KCSAN_SANITIZE_page_alloc.o := n
+# But enable explicit instrumentation for memory barriers.
+KCSAN_INSTRUMENT_BARRIERS := y
 
 # These files are disabled because they produce non-interesting and/or
 # flaky coverage that is not a function of syscall inputs. E.g. slab is out of