io_uring: don't free request to slab

It's not necessary to free the request back to slab when we fail to
get sqe, just move it to state->free_list.

Signed-off-by: Hao Xu <haoxu@linux.alibaba.com>
Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/20210825175856.194299-1-haoxu@linux.alibaba.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>