genksyms: fix syntax error for attribute after 'union'

A longstanding issue with genksyms is that it has hidden syntax errors.

When a syntax error occurs, yyerror() is called. However,
error_with_pos() is a no-op unless the -w option is provided.

You can observe syntax errors by manually passing the -w option.

For example, with CONFIG_MODVERSIONS=y on v6.13-rc1:

    $ make -s KCFLAGS=-D__GENKSYMS__ fs/lockd/svc.i
    $ cat fs/lockd/svc.i | scripts/genksyms/genksyms -w
        [ snip ]
    ./include/net/addrconf.h:35: syntax error

The syntax error occurs in the following code in include/net/addrconf.h:

    union __packed {
            [ snip ]
    };

The issue arises from __packed, which is defined as
__attribute__((__packed__)), immediately after the 'union' keyword.

This commit allows the 'union' keyword to be followed by attributes.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Nicolas Schier <n.schier@avm.de>

scripts/genksyms/parse.y

@@ -236,16 +236,16 @@ type_specifier:
 	   so that it is easier to expand the definition fully later.  */
 	| STRUCT_KEYW attribute_opt IDENT
 		{ remove_node($1); (*$3)->tag = SYM_STRUCT; $$ = $3; }
-	| UNION_KEYW IDENT
-		{ remove_node($1); (*$2)->tag = SYM_UNION; $$ = $2; }
+	| UNION_KEYW attribute_opt IDENT
+		{ remove_node($1); (*$3)->tag = SYM_UNION; $$ = $3; }
 	| ENUM_KEYW IDENT
 		{ remove_node($1); (*$2)->tag = SYM_ENUM; $$ = $2; }
 
 	/* Full definitions of an s/u/e.  Record it.  */
 	| STRUCT_KEYW attribute_opt IDENT class_body
 		{ record_compound($1, $3, $4, SYM_STRUCT); $$ = $4; }
-	| UNION_KEYW IDENT class_body
-		{ record_compound($1, $2, $3, SYM_UNION); $$ = $3; }
+	| UNION_KEYW attribute_opt IDENT class_body
+		{ record_compound($1, $3, $4, SYM_UNION); $$ = $4; }
 	| ENUM_KEYW IDENT enum_body
 		{ record_compound($1, $2, $3, SYM_ENUM); $$ = $3; }
 	/*
@@ -255,7 +255,7 @@ type_specifier:
 		{ add_symbol(NULL, SYM_ENUM, NULL, 0); $$ = $2; }
 	/* Anonymous s/u definitions.  Nothing needs doing.  */
 	| STRUCT_KEYW attribute_opt class_body		{ $$ = $3; }
-	| UNION_KEYW class_body				{ $$ = $2; }
+	| UNION_KEYW attribute_opt class_body		{ $$ = $3; }
 	;
 
 simple_type_specifier: