kernel-doc: fix struct_group_tagged() parsing

kernel-doc emits a warning on struct_group_tagged() if you describe your
struct group member:

include/net/libeth/rx.h:69: warning: Excess struct member 'fp' description in 'libeth_fq'

The code:

/**
 * struct libeth_fq - structure representing a buffer queue
 * @fp: hotpath part of the structure
 * @pp: &page_pool for buffer management
[...]
 */
struct libeth_fq {
	struct_group_tagged(libeth_fq_fp, fp,
		struct page_pool	*pp;
[...]
	);

When a struct_group_tagged() is encountered, we need to build a
`struct TAG NAME;` from it, so that it will be treated as a valid
embedded struct.
Decouple the regex and do the replacement there. As far as I can see,
this doesn't produce any new warnings on the current mainline tree.

Reported-by: Jakub Kicinski <kuba@kernel.org>
Closes: https://lore.kernel.org/netdev/20240405212513.0d189968@kernel.org
Fixes: 50d7bd38

 ("stddef: Introduce struct_group() helper macro")
Signed-off-by: Kees Cook <keescook@chromium.org>
Co-developed-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Signed-off-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240411093208.2483580-1-aleksander.lobakin@intel.com