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/[email protected]
Fixes: fc4d182 ("mm: huge_memory: enable debugfs to split huge pages to any order")
Signed-off-by: Muhammad Usama Anjum <[email protected]>
Reviewed-by: Zi Yan <[email protected]>
Cc: Bill Wendling <[email protected]>
Cc: Justin Stitt <[email protected]>
Cc: Muhammad Usama Anjum <[email protected]>
Cc: Nathan Chancellor <[email protected]>
Cc: Nick Desaulniers <[email protected]>
Cc: Shuah Khan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: musamaanjum

tools/testing/selftests/mm/split_huge_page_test.c

@@ -300,7 +300,7 @@ int create_pagecache_thp_and_fd(const char *testfile, size_t fd_size, int *fd,
 		char **addr)
 {
 	size_t i;
-	int dummy;
+	int __attribute__((unused)) dummy = 0;
 
 	srand(time(NULL));