selftests/ring-buffer: Verify the entire meta-page padding

Improve the ring-buffer meta-page test coverage by checking for the
entire padding region to be 0 instead of just looking at the first 4
bytes.

Cc: linux-kselftest@vger.kernel.org
Link: https://lore.kernel.org/20240910162335.2993310-2-vdonnefort@google.com
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

Author: vdonnefort

tools/testing/selftests/ring-buffer/map_test.c

@@ -231,15 +231,15 @@ TEST_F(map, data_mmap)
 
 	/* Verify meta-page padding */
 	if (desc->meta->meta_page_size > getpagesize()) {
-		void *addr;
-
 		data_len = desc->meta->meta_page_size;
 		data = mmap(NULL, data_len,
 			    PROT_READ, MAP_SHARED, desc->cpu_fd, 0);
 		ASSERT_NE(data, MAP_FAILED);
 
-		addr = (void *)((unsigned long)data + getpagesize());
-		ASSERT_EQ(*((int *)addr), 0);
+		for (int i = desc->meta->meta_struct_len;
+		     i < desc->meta->meta_page_size; i += sizeof(int))
+			ASSERT_EQ(*(int *)(data + i), 0);
+
 		munmap(data, data_len);
 	}
 }