io_uring: verify resv is 0 in ringfd register/unregister

Only allow resv field to be 0 in struct io_uring_rsrc_update user
arguments.

Fixes: e7a6c00d ("io_uring: add support for registering ring file descriptors")
Signed-off-by: Dylan Yudaken <dylany@fb.com>
Link: https://lore.kernel.org/r/20220412163042.2788062-4-dylany@fb.com


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