io_uring: free selected-bufs if error'ed

io_clean_op() may be skipped even if there is a selected io_buffer,
that's because *select_buffer() funcions never set REQ_F_NEED_CLEANUP.

Trigger io_clean_op() when REQ_F_BUFFER_SELECTED is set as well, and
and clear the flag if was freed out of it.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>