XDP flags reference unsoundness

[arctic-alpaca]

Problem:

In the s2n-quic-xdp code, the Ring::flags and Ring::flags_mut methods allow creating shared/mutable references to the flags u32. The flags u32 is shared with the kernel and written to by the kernel. This means creating a reference to the u32 violates Rust's aliasing rules.

Solution:

The flags u32 can be read using pointer std::ptr::read.

[camshaft]

I agree this should be fixed. Thanks for highlighting the issue!