tools/resolve_btfids: Emit warnings and patch zero id for missing symbols

Kernel functions referenced by .BTF_ids may be changed from global to static
and get inlined or get renamed/removed, and thus disappears from BTF.
This causes kernel build failure when resolve_btfids do id patch for symbols
in .BTF_ids in vmlinux. Update resolve_btfids to emit warning messages and
patch zero id for missing symbols instead of aborting kernel build process.

Suggested-by: Andrii Nakryiko <[e-mail]>
Signed-off-by: Hengqi Chen <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Acked-by: Yonghong Song <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/bpf/resolve_btfids/main.c

···
 	sh->sh_addralign = expected;
 
 	if (gelf_update_shdr(scn, sh) == 0) {
-		printf("FAILED cannot update section header: %s\n",
+		pr_err("FAILED cannot update section header: %s\n",
 			elf_errmsg(-1));
 		return -1;
 	}
···
 
 	elf = elf_begin(fd, ELF_C_RDWR_MMAP, NULL);
 	if (!elf) {
+		close(fd);
 		pr_err("FAILED cannot create ELF descriptor: %s\n",
 			elf_errmsg(-1));
 		return -1;
···
 	err = libbpf_get_error(btf);
 	if (err) {
 		pr_err("FAILED: load BTF from %s: %s\n",
-			obj->path, strerror(-err));
+			obj->btf ?: obj->path, strerror(-err));
 		return -1;
 	}
 
···
 	int i;
 
 	if (!id->id) {
-		pr_err("FAILED unresolved symbol %s\n", id->name);
-		return -EINVAL;
+		pr_err("WARN: resolve_btfids: unresolved symbol %s\n", id->name);
 	}
 
 	for (i = 0; i < id->addr_cnt; i++) {
···
 
 	err = 0;
 out:
-	if (obj.efile.elf)
+	if (obj.efile.elf) {
 		elf_end(obj.efile.elf);
-	close(obj.efile.fd);
+		close(obj.efile.fd);
+	}
 	return err;
 }