bpf: Avoid splat in pskb_pull_reason

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

bpf: Avoid splat in pskb_pull_reason

syzkaller builds (CONFIG_DEBUG_NET=y) frequently trigger a debug

hint in pskb_may_pull.

We'd like to retain this debug check because it might hint at integer

overflows and other issues (kernel code should pull headers, not huge

value).

In bpf case, this splat isn't interesting at all: such (nonsensical)

bpf programs are typically generated by a fuzzer anyway.

Do what Eric suggested and suppress such warning.

For CONFIG_DEBUG_NET=n we don't need the extra check because

pskb_may_pull will do the right thing: return an error without the

WARN() backtrace.