selftests/bpf: Fix kmem_cache iterator draining

The closing parentheses around the read syscall is misplaced, causing
single byte reads from the iterator instead of buf sized reads. While
the end result is the same, many more read calls than necessary are
performed.

$ tools/testing/selftests/bpf/vmtest.sh  "./test_progs -t kmem_cache_iter"
145/1   kmem_cache_iter/check_task_struct:OK
145/2   kmem_cache_iter/check_slabinfo:OK
145/3   kmem_cache_iter/open_coded_iter:OK
145     kmem_cache_iter:OK
Summary: 1/3 PASSED, 0 SKIPPED, 0 FAILED

Fixes: a496d0c ("selftests/bpf: Add a test for kmem_cache_iter")
Signed-off-by: T.J. Mercier <[email protected]>
Signed-off-by: Martin KaFai Lau <[email protected]>
Acked-by: Song Liu <[email protected]>
Acked-by: Namhyung Kim <[email protected]>
Link: https://patch.msgid.link/[email protected]

Author: T.J. Mercier

tools/testing/selftests/bpf/prog_tests/kmem_cache_iter.c

@@ -104,7 +104,7 @@ void test_kmem_cache_iter(void)
 		goto destroy;
 
 	memset(buf, 0, sizeof(buf));
-	while (read(iter_fd, buf, sizeof(buf) > 0)) {
+	while (read(iter_fd, buf, sizeof(buf)) > 0) {
 		/* Read out all contents */
 		printf("%s", buf);
 	}