libbpf-tools: Fix cachestat with kernel 5.15

The tool cachestat is broken on kernel 5.15 due to kernel function
renaming. Fix the tool by detecting symbol existence. See #3687 and

Signed-off-by: Hengqi Chen <chenhengqi@outlook.com>

libbpf-tools/cachestat.c

@@ -144,10 +144,29 @@ int main(int argc, char **argv)
 
 	obj = cachestat_bpf__open_and_load();
 	if (!obj) {
-		fprintf(stderr, "failed to open and/or load BPF object\n");
+		fprintf(stderr, "failed to open BPF object\n");
 		return 1;
 	}
 
+	/**
+	 * account_page_dirtied was renamed to folio_account_dirtied
+	 * in kernel commit 203a31516616 ("mm/writeback: Add __folio_mark_dirty()")
+	 */
+	if (fentry_exists("folio_account_dirtied", NULL)) {
+		err = bpf_program__set_attach_target(obj->progs.account_page_dirtied, 0,
+						     "folio_account_dirtied");
+		if (err) {
+			fprintf(stderr, "failed to set attach target\n");
+			goto cleanup;
+		}
+	}
+
+	err = cachestat_bpf__load(obj);
+	if (err) {
+		fprintf(stderr, "failed to load BPF object\n");
+		goto cleanup;
+	}
+
 	if (!obj->bss) {
 		fprintf(stderr, "Memory-mapping BPF maps is supported starting from Linux 5.7, please upgrade.\n");
 		goto cleanup;