io_uring: fix potential use after free on fallback request free

After __io_free_req() puts a ctx ref, it should be assumed that the ctx
may already be gone. However, it can be accessed when putting the
fallback req. Free the req first and then put the ctx.

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