Verifier complexity error

```
{"level":"fatal","msg":"failed to start: daemon creation failed: error while initializing daemon: failed while reinitializing datapath: failed loading eBPF collection into the kernel: program cil_sock4_connect: load program: permission denied: 373: (07) r9 += 4: R9 pointer arithmetic on map_value_or_null prohibited, null-check it first (759 line(s) omitted)","subsys":"daemon"}

2024-07-10T08:01:19.145344750Z Verifier error: program cil_sock6_connect: load program: permission denied: 399: (07) r7 += 4: R7 pointer arithmetic on map_value_or_null prohibited, null-check it first (611 line(s) omitted)

```

Dylan reported -

```
We get a map value back here https://github.com/cilium/cilium/blob/main/bpf/bpf_sock.c#L387
Which we then null-check like we are supposed to do. However, for some reason
the generated byte code does the backend->port access first
so https://github.com/cilium/cilium/blob/main/bpf/bpf_sock.c#L422 or https://github.com/cilium/cilium/blob/main/bpf/bpf_sock.c#L434
We don't have the actual bytecode at the moment, but what I suspect
 is happening is that the compiler is computing the offset of backend->port
 without accessing the memory before the null check.
 ```

Add a barrier call so that compiler doesn't reorder the memory accesses.

Signed-off-by: Aditi Ghag <aditi@cilium.io>