io_uring: clean up a type in io_uring_register_get_file()

io_uring: clean up a type in io_uring_register_get_file()

Originally "fd" was unsigned int but it was changed to int when we pulled
this code into a separate function in commit 0b6d253e084a
("io_uring/register: provide helper to get io_ring_ctx from 'fd'").  This
doesn't really cause a runtime problem because the call to
array_index_nospec() will clamp negative fds to 0 and nothing else uses
the negative values.

Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/r/6f6cb630-079f-4fdf-bf95-1082e0a3fc6e@stanley.mountain
Signed-off-by: Jens Axboe <axboe@kernel.dk>