io_uring: recycle provided before arming poll

We currently have a race where we recycle the selected buffer if poll
returns IO_APOLL_OK. But that's too late, as the poll could already be
triggering or have triggered. If that race happens, then we're putting a
buffer that's already being used.

Fix this by recycling before we arm poll. This does mean that we'll
sometimes almost instantly re-select the buffer, but it's rare enough in
testing that it should not pose a performance issue.

Fixes: b1c62645 ("io_uring: recycle provided buffers if request goes async")
Signed-off-by: Jens Axboe <axboe@kernel.dk>