io_uring: simplify file ref tracking in submission state

Currently, file refs in struct io_submit_state are tracked with 2 vars:
@has_refs -- how many refs were initially taken
@used_refs -- number of refs used

Replace it with a single variable counting how many refs left at the
current moment.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>