io_uring/register: have io_parse_restrictions() return number of ops

io_uring/register: have io_parse_restrictions() return number of ops

Rather than return 0 on success, return >= 0 for success, where the
return value is that number of parsed entries. As before, any < 0
return is an error.

Reviewed-by: Gabriel Krisman Bertazi <krisman@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>