selftests: filesystems: fix warn_unused_result build warnings

Add return value checks for read & write calls in test_listmount_ns
function. This patch resolves below compilation warnings:

```
statmount_test_ns.c: In function ‘test_listmount_ns’:

statmount_test_ns.c:322:17: warning: ignoring return value of ‘write’
declared with attribute ‘warn_unused_result’ [-Wunused-result]

statmount_test_ns.c:323:17: warning: ignoring return value of ‘read’
declared with attribute ‘warn_unused_result’ [-Wunused-result]
```
Signed-off-by: Abhinav Jain <jain.abhinav177@gmail.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/filesystems/statmount/statmount_test_ns.c

@@ -319,8 +319,11 @@ static void test_listmount_ns(void)
 		 * Tell our parent how many mounts we have, and then wait for it
 		 * to tell us we're done.
 		 */
-		write(child_ready_pipe[1], &nr_mounts, sizeof(nr_mounts));
-		read(parent_ready_pipe[0], &cval, sizeof(cval));
+		if (write(child_ready_pipe[1], &nr_mounts, sizeof(nr_mounts)) !=
+					sizeof(nr_mounts))
+			ret = NSID_ERROR;
+		if (read(parent_ready_pipe[0], &cval, sizeof(cval)) != sizeof(cval))
+			ret = NSID_ERROR;
 		exit(NSID_PASS);
 	}