iommu/selftest: prevent use of uninitialized variable

Fix to avoid the usage of the `res` variable uninitialized in the
following macro expansions.

It solves the following warning:
In function ‘iommufd_viommu_vdevice_alloc’,
  inlined from ‘wrapper_iommufd_viommu_vdevice_alloc’ at iommufd.c:2889:1:
../kselftest_harness.h:760:12: warning: ‘ret’ may be used uninitialized [-Wmaybe-uninitialized]
  760 |   if (!(__exp _t __seen)) { \
      |      ^
../kselftest_harness.h:513:9: note: in expansion of macro ‘__EXPECT’
  513 |   __EXPECT(expected, #expected, seen, #seen, ==, 1)
      |   ^~~~~~~~
iommufd_utils.h:1057:9: note: in expansion of macro ‘ASSERT_EQ’
 1057 |   ASSERT_EQ(0, _test_cmd_trigger_vevents(self->fd, dev_id, nvevents))
      |   ^~~~~~~~~
iommufd.c:2924:17: note: in expansion of macro ‘test_cmd_trigger_vevents’
 2924 |   test_cmd_trigger_vevents(dev_id, 3);
      |   ^~~~~~~~~~~~~~~~~~~~~~~~

The issue can be reproduced, building the tests, with the command: make -C
tools/testing/selftests TARGETS=iommu

Link: https://patch.msgid.link/r/[email protected]
Fixes: 97717a1 ("iommufd/selftest: Add IOMMU_VEVENTQ_ALLOC test coverage")
Signed-off-by: Alessandro Zanni <[email protected]>
Signed-off-by: Jason Gunthorpe <[email protected]>

Author: Alez87

tools/testing/selftests/iommu/iommufd_utils.h

@@ -1042,15 +1042,13 @@ static int _test_cmd_trigger_vevents(int fd, __u32 dev_id, __u32 nvevents)
 			.dev_id = dev_id,
 		},
 	};
-	int ret;
 
 	while (nvevents--) {
-		ret = ioctl(fd, _IOMMU_TEST_CMD(IOMMU_TEST_OP_TRIGGER_VEVENT),
-			    &trigger_vevent_cmd);
-		if (ret < 0)
+		if (!ioctl(fd, _IOMMU_TEST_CMD(IOMMU_TEST_OP_TRIGGER_VEVENT),
+			    &trigger_vevent_cmd))
 			return -1;
 	}
-	return ret;
+	return 0;
 }
 
 #define test_cmd_trigger_vevents(dev_id, nvevents) \