tun: revert fix group permission check

tun: revert fix group permission check

This reverts commit 3ca459eaba1bf96a8c7878de84fa8872259a01e3.

The blamed commit caused a regression when neither tun->owner nor
tun->group is set. This is intended to be allowed, but now requires
CAP_NET_ADMIN.

Discussion in the referenced thread pointed out that the original
issue that prompted this patch can be resolved in userspace.

The relaxed access control may also make a device accessible when it
previously wasn't, while existing users may depend on it to not be.

This is a clean pure git revert, except for fixing the indentation on
the gid_valid line that checkpatch correctly flagged.

Fixes: 3ca459eaba1b ("tun: fix group permission check")
Link: https://lore.kernel.org/netdev/CAFqZXNtkCBT4f+PwyVRmQGoT3p1eVa01fCG_aNtpt6dakXncUg@mail.gmail.com/
Signed-off-by: Willem de Bruijn <willemb@google.com>
Cc: Ondrej Mosnacek <omosnace@redhat.com>
Cc: Stas Sergeev <stsp2@yandex.ru>
Link: https://patch.msgid.link/20250204161015.739430-1-willemdebruijn.kernel@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>