io_wait: run the fd-map consistency check only with EXTRA_DEBUG

[Lt-Flash]

Summary

check_io_data() in io_wait.h is a consistency pass over the reactor's fd map — every one of the max_fd_no entries — and it runs on every io_watch_add() and io_watch_del(). The macro is defined unconditionally, so the plain build pays it too.

That makes each async registration or removal O(reactor size), i.e. O(open_files_limit). Measured on a 16-core host with a 21k-entry reactor (open_files_limit 65536, reactor shrunk to 20971 by the pkg budget): ~0.6 ms per call. A module doing a few thousand async operations a second — here cross-node cache pulls through the async framework, but any async DB or REST traffic takes the same path — spent about two cores in it: io_watch_add/io_watch_del were the largest user-space symbols of a perf profile at 13.6% of all CPU, and the receive buffers of the SIP sockets overflowed (129 UdpRcvbufErrors over a run) because the workers were busy walking the map.

Change

The check is compiled only with EXTRA_DEBUG; the plain build keeps check_error cleared and does nothing. No behaviour change otherwise.

Effect

Same workload, same host, before → after: async-heavy worker CPU down by the two cores, UDP receive drops 129 → 0, request p95 in the early part of the run 0.97 → 0.91 ms; the check still runs in EXTRA_DEBUG builds.

Built with gcc and clang under -Werror.