Fix landlock06 test

Initialize the flag variable, so that FIOASYNC does not spuriously fail
with ENOTTY (/dev/zero has no support for FASYNC).

Signed-off-by: Andreas Schwab <schwab@suse.de>
Reviewed-by: Andrea Cervesato <andrea.cervesato@suse.com>

testcases/kernel/syscalls/landlock/landlock06.c

@@ -28,7 +28,7 @@ static void run(void)
 	if (SAFE_FORK())
 		return;
 
-	int flag;
+	int flag = 0;
 	size_t sz = 0;
 
 	TST_EXP_PASS(ioctl(file_fd, FIONREAD, &sz));