kcsan: test: Initialize dummy variable

kcsan: test: Initialize dummy variable

Newer compiler versions rightfully point out:

 kernel/kcsan/kcsan_test.c:591:41: error: variable 'dummy' is
 uninitialized when passed as a const pointer argument here
 [-Werror,-Wuninitialized-const-pointer]
   591 |         KCSAN_EXPECT_READ_BARRIER(atomic_read(&dummy), false);
       |                                                ^~~~~
 1 error generated.

Although this particular test does not care about the value stored in
the dummy atomic variable, let's silence the warning.

Link: https://lkml.kernel.org/r/CA+G9fYu8JY=k-r0hnBRSkQQrFJ1Bz+ShdXNwC1TNeMt0eXaxeA@mail.gmail.com
Fixes: 8bc32b348178 ("kcsan: test: Add test cases for memory barrier instrumentation")
Reported-by: Linux Kernel Functional Testing <lkft@linaro.org>
Reviewed-by: Alexander Potapenko <glider@google.com>
Signed-off-by: Marco Elver <elver@google.com>