Move io_fallback_req_func() to kill yet another forward declaration.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/d0a8f9d9a0057ed761d6237167d51c9378798d2d.1628536684.git.asml.silence@gmail.com


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