io_uring backend is too ENOMEM-y in older kernels, leading to unwanted errors

[xemul]

There's a fix in Linux that addresses that (https://github.com/torvalds/linux/commit/182db972c9568dc530b2f586a2f82dfd039d9f2a), but for older kernels it's better to ban io-uring backend and fall-back to AIO one