eventpoll: rename epi->next and txlist for clarity

eventpoll: rename epi->next and txlist for clarity

Two list-related names were confusing in isolation:

  struct epitem::next
    A singly-linked link slot used only when an epi is queued on
    ep->ovflist during an ep_start_scan/ep_done_scan window. The
    bare name "next" suggests a generic list link and doesn't say
    which list it belongs to.

  txlist
    The caller-local list_head used by ep_send_events() and
    __ep_eventpoll_poll() to hold the batch of items stolen from
    ep->rdllist for the current scan. "txlist" ("transmission
    list") is abbreviated and overloaded: it doesn't distinguish
    itself from ep->rdllist or ep->ovflist at a glance.

Rename for what each actually is:

  struct epitem::next   -> struct epitem::ovflist_next
  local txlist          -> scan_batch

With these in place:
  - epi->ovflist_next reads as "this is the ep->ovflist link slot",
    matching the rdllink pattern above it.
  - scan_batch reads as "the batch currently being scanned", clearly
    distinct from rdllist (canonical ready list) and ovflist
    (scan-window overflow).

ep->rdllist and ep->ovflist struct field names are preserved -- they
are long-standing interface-facing identifiers, and the new inline
helpers (ep_is_scanning, epi_on_ovflist, ...) already hide the
sentinel semantics at call sites.

No functional change.

Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
Link: https://patch.msgid.link/20260424-work-epoll-rework-v1-15-249ed00a20f3@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>