gpt-auto-generator: fix error propagation in add_root_mount()

When generator_write_initrd_root_device_deps() fails, the error was
swallowed by returning 0 (success) instead of r. The two subsequent
calls in the same block correctly return r on failure.

Signed-off-by: dongshengyuan <dongshengyuan@uniontech.com>
Co-developed-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/gpt-auto-generator/gpt-auto-generator.c

@@ -882,7 +882,7 @@ static int add_root_mount(void) {
         if (in_initrd()) {
                 r = generator_write_initrd_root_device_deps(arg_dest_late, bdev);
                 if (r < 0)
-                        return 0;
+                        return r;
 
                 r = add_root_cryptsetup();
                 if (r < 0)