Reincarnation of an old patch that replaces a list in struct
io_compl_batch with an array. It's needed to avoid hooking requests via
their compl.list, because it won't be always available in the future.

It's also nice to split io_submit_flush_completions() to avoid free
under locks and remove unlock/lock with a long comment describing when
it can be done.
Signed-off-by: NPavel Begunkov <asml.silence@gmail.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>