eventpoll: hoist CTL_ADD scratch state into struct ep_ctl_ctx

eventpoll: hoist CTL_ADD scratch state into struct ep_ctl_ctx

Three globals were shared between the loop check and the path check
paths: tfile_check_list (chain of epitems_head to walk afterwards),
path_count[] (per-depth wakeup-path tally) and inserting_into
(cycle-detection sentinel). All three are scratch state used only
during a single EPOLL_CTL_ADD full_check, yet they sit at file
scope and rely on epnested_mutex for exclusion.

The area has had three bugs in the last year -- CVE-2025-38349,
f2e467a48287 ("eventpoll: Fix semi-unbounded recursion"), and
fdcfce93073d ("eventpoll: Fix integer overflow in
ep_loop_check_proc()") -- all rooted in the shared-mutable-global
pattern being hard to reason about.

Collect the three into a stack-allocated struct ep_ctl_ctx:

    struct ep_ctl_ctx {
        struct eventpoll   *inserting_into;
        struct epitems_head *tfile_check_list;
        int                 path_count[PATH_ARR_SIZE];
    };

do_epoll_ctl() zero-initializes one on its stack and plumbs it
through ep_ctl_lock() / ep_ctl_unlock() / ep_insert() /
ep_register_epitem() / list_file() / ep_loop_check() /
ep_loop_check_proc() / reverse_path_check() /
reverse_path_check_proc() / path_count_inc() / path_count_init() /
clear_tfile_check_list(). Non-nested inserts leave the ctx zeroed
and skip the machinery entirely.

With the scratch state in ctx:
  - tfile_check_list no longer has an EP_UNACTIVE_PTR sentinel --
    NULL is the obvious "empty" value and the zero-init handles it
    for free;
  - path_count[] is no longer an array global that could be touched
    in unexpected orderings;
  - inserting_into is scoped to the exact call that set it.

loop_check_gen stays as a file-scope monotonic counter, because the
stamp left on ep->gen by a completed walk must not equal the stamp
of a future walk -- something a stack-local value cannot guarantee
across calls. It remains protected by epnested_mutex for the bump
and read lockless for the "do we need a full check" trigger in
ep_ctl_lock().

Every bail-out that existed before (the ELOOP on cycle, the path
limit check, the unbounded-recursion cap, the +1 overflow guard) is
preserved verbatim; only the data they operate on moved from file
scope to the stack ctx.

No functional change.

Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
Link: https://patch.msgid.link/20260424-work-epoll-rework-v1-17-249ed00a20f3@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>