io_uring/rw: reinstate thread check for retries

Allowing retries for everything is arguably the right thing to do, now
that every command type is async read from the start. But it's exposed a
few issues around missing check for a retry (which cca65713 exposed),
and the fixup commit for that isn't necessarily 100% sound in terms of
iov_iter state.

For now, just revert these two commits. This unfortunately then re-opens
the fact that -EAGAIN can get bubbled to userspace for some cases where
the kernel very well could just sanely retry them. But until we have all
the conditions covered around that, we cannot safely enable that.

This reverts commit df604d2a.
This reverts commit cca65713.

Signed-off-by: Jens Axboe <axboe@kernel.dk>