bpf: Make sure internal and UAPI bpf_redirect flags don't overlap

In the Linux kernel, the following vulnerability has been resolved:

bpf: Make sure internal and UAPI bpf_redirect flags don't overlap

The bpf_redirect_info is shared between the SKB and XDP redirect paths,

and the two paths use the same numeric flag values in the ri->flags

field (specifically, BPF_F_BROADCAST == BPF_F_NEXTHOP). This means that

if skb bpf_redirect_neigh() is used with a non-NULL params argument and,

subsequently, an XDP redirect is performed using the same

bpf_redirect_info struct, the XDP path will get confused and end up

crashing, which syzbot managed to trigger.

With the stack-allocated bpf_redirect_info, the structure is no longer

shared between the SKB and XDP paths, so the crash doesn't happen

anymore. However, different code paths using identically-numbered flag

values in the same struct field still seems like a bit of a mess, so

this patch cleans that up by moving the flag definitions together and

redefining the three flags in BPF_F_REDIRECT_INTERNAL to not overlap

with the flags used for XDP. It also adds a BUILD_BUG_ON() check to make

sure the overlap is not re-introduced by mistake.