selftests: mm: fix unused and uninitialized variable warning

Fix the warnings by initializing and marking the variable as unused.
I've caught the warnings by using clang.

split_huge_page_test.c:303:6: warning: variable 'dummy' set but not used [-Wunused-but-set-variable]
  303 |         int dummy;
      |             ^
split_huge_page_test.c:343:3: warning: variable 'dummy' is uninitialized when used here [-Wuninitialized]
  343 |                 dummy += *(*addr + i);
      |                 ^~~~~
split_huge_page_test.c:303:11: note: initialize the variable 'dummy' to silence this warning
  303 |         int dummy;
      |                  ^
      |                   = 0
2 warnings generated.

Link: https://lkml.kernel.org/r/20240416162658.3353622-1-usama.anjum@collabora.com
Fixes: fc4d1823

 ("mm: huge_memory: enable debugfs to split huge pages to any order")
Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Cc: Bill Wendling <morbo@google.com>
Cc: Justin Stitt <justinstitt@google.com>
Cc: Muhammad Usama Anjum <usama.anjum@collabora.com>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>