io_uring: make submission ref putting consistent

The rule is simple, any async handler gets a submission ref and should
put it at the end. Make them all follow it, and so more consistent.

This is a preparation patch, and as io_wq_assign_next() currently won't
ever work, this doesn't care to use io_put_req_find_next() instead of
io_put_req().

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>

refcount_inc_not_zero() -> refcount_inc() fix.

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