drivers: lkdtm: fix clang -Wformat warning

When building with Clang we encounter the following warning
(ARCH=hexagon + CONFIG_FRAME_WARN=0):
| ../drivers/misc/lkdtm/bugs.c:107:3: error: format specifies type
| 'unsigned long' but the argument has type 'int' [-Werror,-Wformat]
|                 REC_STACK_SIZE, recur_count);
|                 ^~~~~~~~~~~~~~

Cast REC_STACK_SIZE to `unsigned long` to match format specifier `%lu`
as well as maintain symmetry with `#define REC_STACK_SIZE
(_AC(CONFIG_FRAME_WARN, UL) / 2)`.

Link: https://github.com/ClangBuiltLinux/linux/issues/378


Reported-by: Nathan Chancellor <nathan@kernel.org>
Suggested-by: Nathan Chancellor <nathan@kernel.org>
Suggested-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Justin Stitt <justinstitt@google.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Acked-by: Kees Cook <keescook@chromium.org>
Fixes: 24cccab4 ("lkdtm/bugs: Adjust recursion test to avoid elision")
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220721215706.4153027-1-justinstitt@google.com