genksyms: Fix parsing a declarator with a preceding attribute

After commit 07919126ecfc ("netfilter: annotate NAT helper hook pointers
with __rcu"), genksyms fails to parse the __rcu annotation when building
with CONFIG_DEBUG_INFO_BTF=y, CONFIG_PAHOLE_HAS_BTF_TAG=y, and a version
of clang that supports btf_type_tag.

$ clang --version | head -1
ClangBuiltLinux clang version 22.1.0 (https://github.com/llvm/llvm-project.git 4434dabb69916856b824f68a64b029c67175e532)

$ cat kernel/configs/repro.config
CONFIG_BPF_SYSCALL=y
CONFIG_MODVERSIONS=y
# CONFIG_DEBUG_INFO_NONE is not set
CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT=y
CONFIG_DEBUG_INFO_BTF=y

$ make -skj"$(nproc)" ARCH=x86_64 LLVM=1 mrproper defconfig repro.config all
WARNING: modpost: EXPORT symbol "nf_nat_ftp_hook" [vmlinux] version generation failed, symbol will not be versioned.
...
WARNING: modpost: EXPORT symbol "nf_nat_irc_hook" [vmlinux] version generation failed, symbol will not be versioned.
...

genksyms falls over parsing the __rcu attribute in the declarator:

# Kernel reproducer
$ make -skj"$(nproc)" ARCH=x86_64 KCFLAGS=-D__GENKSYMS__ LLVM=1 net/netfilter/nf_conntrack_ftp.i

$ scripts/genksyms/genksyms -w <net/netfilter/nf_conntrack_ftp.i &| rg 'syntax error'
include/linux/netfilter/nf_conntrack_ftp.h:29: syntax error
net/netfilter/nf_conntrack_ftp.c:46: syntax error

# Trivial reproducer
$ cat test.c
int (*func)(void *foo, int bar);
int (__attribute__((btf_type_tag("rcu"))) *func_with_attr)(void *foo, int bar);

$ scripts/genksyms/genksyms -w <test.c
<stdin>:2: syntax error

Optionally allow an attribute to precede a declarator to resolve this
error and properly generate symbol versions.

Fixes: 07919126ecfc ("netfilter: annotate NAT helper hook pointers with __rcu")
Link: https://patch.msgid.link/[e-mail]
Tested-by: Nicolas Schier <[e-mail]>
Reviewed-by: Nicolas Schier <[e-mail]>
Signed-off-by: Nathan Chancellor <[e-mail]>

scripts/genksyms/parse.y

···
 		{ $$ = $4; }
 	| direct_declarator BRACKET_PHRASE
 		{ $$ = $2; }
-	| '(' declarator ')'
-		{ $$ = $3; }
+	| '(' attribute_opt declarator ')'
+		{ $$ = $4; }
 	;
 
 /* Nested declarators differ from regular declarators in that they do