bpf, verifier: reject xadd on flow key memory

We should not enable xadd operation for flow key memory if not
needed there anyway. There is no such issue as described in the
commit f37a8cb8 ("bpf: reject stores into ctx via st and xadd")
since there's no context rewriter for flow keys today, but it
also shouldn't become part of the user facing behavior to allow
for it. After patch:

  0: (79) r7 = *(u64 *)(r1 +144)
  1: (b7) r3 = 4096
  2: (db) lock *(u64 *)(r7 +0) += r3
  BPF_XADD stores into R7 flow_keys is not allowed

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>