io_uring: abolish old io_put_file()

io_put_file() doesn't do a good job at generating a good code. Inline
it, so we can check REQ_F_FIXED_FILE first, prioritising FIXED_FILE case
over requests without files, and saving a memory load in that case.

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