tomoyo: fix clang pointer arithmetic warning

clang warns about additions on NULL pointers being undefined in C:

security/tomoyo/securityfs_if.c:226:59: warning: arithmetic on a null pointer treated as a cast from integer to pointer is a GNU extension [-Wnull-pointer-arithmetic]
        securityfs_create_file(name, mode, parent, ((u8 *) NULL) + key,

Change the code to instead use a cast through uintptr_t to avoid
the warning.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>

security/tomoyo/securityfs_if.c

@@ -131,8 +131,8 @@ static const struct file_operations tomoyo_self_operations = {
  */
 static int tomoyo_open(struct inode *inode, struct file *file)
 {
-	const int key = ((u8 *) file_inode(file)->i_private)
-		- ((u8 *) NULL);
+	const u8 key = (uintptr_t) file_inode(file)->i_private;
+
 	return tomoyo_open_control(key, file);
 }
 
@@ -223,7 +223,7 @@ static const struct file_operations tomoyo_operations = {
 static void __init tomoyo_create_entry(const char *name, const umode_t mode,
 				       struct dentry *parent, const u8 key)
 {
-	securityfs_create_file(name, mode, parent, ((u8 *) NULL) + key,
+	securityfs_create_file(name, mode, parent, (void *) (uintptr_t) key,
 			       &tomoyo_operations);
 }