io_uring: optimise iowq refcounting

If a requests is forwarded into io-wq, there is a good chance it hasn't
been refcounted yet and we can save one req_ref_get() by setting the
refcount number to the right value directly.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/2d53f4449faaf73b4a4c5de667fc3c176d974860.1628981736.git.asml.silence@gmail.com


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