genksyms: fix syntax error for attribute after abstact_declarator

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__ kernel/module/main.i
    $ cat kernel/module/main.i | scripts/genksyms/genksyms -w
        [ snip ]
    kernel/module/main.c:97: syntax error

The syntax error occurs in the following code in kernel/module/main.c:

    static void __mod_update_bounds(enum mod_mem_type type __maybe_unused, void *base,
                                    unsigned int size, struct mod_tree_root *tree)
    {
            [ snip ]
    }

The issue arises from __maybe_unused, which is defined as
__attribute__((__unused__)).

This commit allows direct_abstract_declarator to be followed with
attributes.

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

Author: masahir0y

scripts/genksyms/parse.y

@@ -383,8 +383,8 @@ abstract_declarator:
 	ptr_operator
 	| ptr_operator abstract_declarator
 		{ $$ = $2 ? $2 : $1; }
-	| direct_abstract_declarator
-		{ $$ = $1; dont_want_type_specifier = false; }
+	| direct_abstract_declarator attribute_opt
+		{ $$ = $2; dont_want_type_specifier = false; }
 	;
 
 direct_abstract_declarator: