io_uring: use task_work for links if possible

Currently links are always done in an async fashion, unless we catch them
inline after we successfully complete a request without having to resort
to blocking. This isn't necessarily the most efficient approach, it'd be
more ideal if we could just use the task_work handling for this.

Outside of saving an async jump, we can also do less prep work for these
kinds of requests.

Running dependent links from the task_work handler yields some nice
performance benefits. As an example, examples/link-cp from the liburing
repository uses read+write links to implement a copy operation. Without
this patch, the a cache fold 4G file read from a VM runs in about 3
seconds:

$ time examples/link-cp /data/file /dev/null

real	0m2.986s
user	0m0.051s
sys	0m2.843s

and a subsequent cache hot run looks like this:

$ time examples/link-cp /data/file /dev/null

real	0m0.898s
user	0m0.069s
sys	0m0.797s

With this patch in place, the cold case takes about 2.4 seconds:

$ time examples/link-cp /data/file /dev/null

real	0m2.400s
user	0m0.020s
sys	0m2.366s

and the cache hot case looks like this:

$ time examples/link-cp /data/file /dev/null

real	0m0.676s
user	0m0.010s
sys	0m0.665s

As expected, the (mostly) cache hot case yields the biggest improvement,
running about 25% faster with this change, while the cache cold case
yields about a 20% increase in performance. Outside of the performance
increase, we're using less CPU as well, as we're not using the async
offload threads at all for this anymore.

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

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