buf_page_io_complete(): Do not test bpage for NULL, because
it is declared (and always passed) as nonnull.

buf_flush_batch(): Remove the constant local variable count=0.

fil_ibd_load(): Use magic comment to suppress -Wimplicit-fallthrough.

ut_stage_alter_t::inc(ulint): Disable references to an unused parameter.

lock_queue_validate(), sync_array_find_thread(), rbt_check_ordering():
Define only in debug builds.