selftests/bpf: Fix memory leak in test_ima

The allocated ring buffer is never freed, do so in the cleanup path.

Fixes: f446b570 ("bpf/selftests: Update the IMA test to use BPF ring buffer")
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20211028063501.2239335-9-memxor@gmail.com

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

@@ -43,7 +43,7 @@ static int process_sample(void *ctx, void *data, size_t len)
 void test_test_ima(void)
 {
 	char measured_dir_template[] = "/tmp/ima_measuredXXXXXX";
-	struct ring_buffer *ringbuf;
+	struct ring_buffer *ringbuf = NULL;
 	const char *measured_dir;
 	char cmd[256];
 
@@ -85,5 +85,6 @@ void test_test_ima(void)
 	err = system(cmd);
 	CHECK(err, "failed to run command", "%s, errno = %d\n", cmd, errno);
 close_prog:
+	ring_buffer__free(ringbuf);
 	ima__destroy(skel);
 }