kbuild/modpost: improve warnings if symbol is unknown

If we cannot determine the symbol then print
(unknown) to hint the reader that we failed to
find the symbol.
This happens with REL relocation records
in arm object files.
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

scripts/mod/modpost.c

@@ -613,7 +613,7 @@ static const char *sym_name(struct elf_info *elf, Elf_Sym *sym)
 	if (sym)
 		return elf->strtab + sym->st_name;
 	else
-		return "";
+		return "(unknown)";
 }
 
 static const char *sec_name(struct elf_info *elf, int shndx)
@@ -1102,7 +1102,7 @@ static int is_function(Elf_Sym *sym)
 	if (sym)
 		return ELF_ST_TYPE(sym->st_info) == STT_FUNC;
 	else
-		return 0;
+		return -1;
 }
 
 /*
@@ -1120,10 +1120,17 @@ static void report_sec_mismatch(const char *modname, enum mismatch mismatch,
 {
 	const char *from, *from_p;
 	const char *to, *to_p;
-	from = from_is_func ? "function" : "variable";
-	from_p = from_is_func ? "()" : "";
-	to = to_is_func ? "function" : "variable";
-	to_p = to_is_func ? "()" : "";
+
+	switch (from_is_func) {
+	case 0: from = "variable"; from_p = "";   break;
+	case 1: from = "function"; from_p = "()"; break;
+	default: from = "(unknown reference)"; from_p = ""; break;
+	}
+	switch (to_is_func) {
+	case 0: to = "variable"; to_p = "";   break;
+	case 1: to = "function"; to_p = "()"; break;
+	default: to = "(unknown reference)"; to_p = ""; break;
+	}
 
 	sec_mismatch_count++;
 	if (!sec_mismatch_verbose)
@@ -1137,7 +1144,7 @@ static void report_sec_mismatch(const char *modname, enum mismatch mismatch,
 	switch (mismatch) {
 	case TEXT_TO_INIT:
 		fprintf(stderr,
-		"The function %s %s() references\n"
+		"The function %s%s() references\n"
 		"the %s %s%s%s.\n"
 		"This is often because %s lacks a %s\n"
 		"annotation or the annotation of %s is wrong.\n",