io_uring: remove dead 'ctx' argument and move forward declaration

We don't use 'ctx' at all in io_sq_thread_drop_mm(), it just works
on the mm of the current task. Drop the argument.

Move io_file_put_work() to where we have the other forward declarations
of functions.

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