[PATCH] landlock: Drop __counted_by from landlock_domain.layers

[Nguyen Ngoc Thang]

Hi Nathan, Alex,

Thanks for the pointers. I wasn't aware of the earlier thread, so please
disregard my patch in favor of the one from Qualcomm and the Clang
version gating of __counted_by.

To answer Alex's question: I only tested with Clang 22.1.8 (Debian
22.1.8-1~deb13u4, same as the syzbot build). I did not try Clang 23 or
later, so I can't say whether it is fixed there.

Thanks,
Nguyen